Touring bicycles are very highly versatile bikes made to go long distances ("touring," as cyclists call it), fairly efficiently, fairly comfortably, carrying moderate to heavy amounts of rack-mounted cargo, and in variable terrain and weather conditions. They are generally meant with on-road riding in mind, and are usually thought of as a type of road bike, but true touring bikes are fully capable of moderate off-roading, functioning essentially as less-aggressive but equally-strong cyclocross bikes. They can arguably be subdivided into "loaded" touring bikes (aka "true" touring bikes), which are essentially designed for long-distance self-contained trips (ie, carrying full camping gear and a week of food), and "sport" or "light" touring bikes, which have many similar features but are generally lighter and used for unloaded long-distance day trips or "credit card touring"(trips where you sleep in hotels and eat at restraunts). However, it's important to note that many feel the term "touring bike" should technically refer to the true loaded touring bike, most of which deviate in important frame features and design goals fairly little. The bikes designed and/or marketed as light or sport tourers inhabit a wide spectrum of significantly different features and trade-offs, with some being very close to racing bikes and others being nearly the same as loaded touring bikes. Note that it's common for someone to use and refer to a bike as their touring bike, even though it's not technically a touring bike at all, leading to some confusion. Also note that the industry trend for all mainstream road bikes to essentially have racing features has caused bikes that would once have been referred to as "road bikes" to now be called "light touring bikes," since mainstream road bikes used to be designed with a moderate amount of versatility in mind. So essentially, the way the term is currently used, its meaning is highly arbitrary beyond the fact that it describes a bicycle made to go long distances, with some people feeling it should stop there, and other people feeling it should technically be used to describe a true, loaded touring bike.
In addition to being the optimal bikes for loaded long-distance travel, loaded touring bikes are probably the optimal all-around daily rider and commuter for many people. There's not really a more versatile type of frame out there. They're in the same approximate speed category of any road bike, and much faster than mountain bikes and hybrids (hybrids in many ways are just flat-barred touring bikes, usually on the cheap side), although they trade some speed for comfort and strength. They're reliable and can handle whatever moderate off-road stuff you want to do. They can fit fenders and wide tires. They can carry weight well. And they can do it all for years and years. If you're a serious cyclist but can only afford one nice bike, a touring bike is an excellent choice.
Loaded touring bikes were extremely popular during the mid-eighties, as the bike industry experienced a random rise in the popularity of bike touring. Since then, interest has gradually tapered off, and relatively few thoroughbred stock
touring bikes are currently manufactured today. They remain a popular choice among those in the know, though.
Here are the design considerations/features for touring bikes:
Frame/fork Geometry: Touring bikes are characterized by having seat and head tube angles somewhere on the shallow, non-aggressive side, with a large amount of fork rake. This causes less road shock to be transmitted directly to the rider. The slack head tube angle also makes the bike steer gradually as opposed to quickly. Touring bikes have low bottom bracket heights because this helps with stability. Touring bikes have a long wheelbase, granted by their long chainstays and fork rakes/head angles, which helps give enough clearance between heels and pannier bags, helps with steering stability, and also helps transmit less road shock to the rider.
Frame materials: Steel is the classic material for touring bikes, with a fairly low amount of deviation in the current era. (Most steel used for touring bikes would be one of the various Chro-Moly alloys popular for bike frames). There are a few titanium tourers out there, most of which don't have a true loaded touring feature set, and a few aluminum ones (Cannondale makes very popular loaded touring bikes). Currently there are no carbon fiber loaded touring bikes, although there are some that are marketed as "sport touring." Without getting into a discussion of frame materials, one of the reasons why steel is popular is its high repairability. In the case of frame damage or failure, many repairs can theoretically be made by any local who knows how to braze, and alignment issues resulting from a crash can be repaired by cold-setting, unlike other materials. This is particularly important to cyclists planning on going through less industrialized areas. Also, steel has the reputation of being reliable and is indeed the material most conducive to making bike frames that will last decades under constant, hard use, but ti and aluminum can also compete on these terms if the frame is designed correctly. Very few companies make aluminum loaded touring bikes though, and many people hold that aluminum's lack of a fatigue limit make it inappropriate for loaded touring. (A metal's fatigue limit is the amount of stress it can operate under indefinitely without leading to weakening and eventual failure; steel and ti have fairly high ones, while aluminum doesn't have one at all, meaning it will theoretically invariably fail one day under if under constant use). Also, many feel that frame material has a great impact on comfort (the typical dogma is that steel and ti are "springy" while aluminum is "stiff"), but this is basically nonsense propagated by the bike industry. A frame's ride comfort qualities have almost exclusively to do with frame geometry choices. Stock aluminum loaded tourers tend to have steel forks, and no loaded tourers come with carbon forks.
Almost all touring bikes are high enough quality to have at least double-butted main tubes, with many having triple- or quad- butted ones. In order to handle rough use and load-carrying ability, heavier tubing gauges are used, leading to a heavier frame. (Loaded touring frames really have a lot in common with many rigid steel mountain bike frames).
Braze-ons: This is one of the areas where the difference between true touring bikes and everything else is most critical and apparent. Loaded touring bikes have at least 2 and usually 3 bottle cage mountings, a pump peg, rear rack braze-ons (if it doesn't have this and the manufacturer is calling it any sort of touring bike, be very wary), mid-fork front rack braze-ons, double front and rear rack/fender dropout eyelets, and cantilever brake bosses (another biggie where if it doesn't have it, it's only very arbitrarily a touring bike). Many also have some sort of spare spoke carrier and some other miscellaneous handy stuff, but these aren't critical. The non-loaded varieties of touring bike are a mixed bag as far as braze-ons go.
Brakes and fender/tire clearances: Most touring bikes use cantilever brakes, either the traditional type or the currently popular linear-pull (v-brake) type. One reason is these give more braking power (higher mechanical advantage) than the various types of caliper brake (the other family of brakes that use the wheel rim as a braking surface), which is useful for doing things like taking 60 pounds of extra weight down a mountain pass. However, another extremely important reason is that canti brakes allow for large amounts of clearance between wheel and frame/fork, which allows one to easily use both fenders and wider tires if desired. There are limits to how much clearance can be had underneath a caliper brake, and the most popular modern ones in particular essentially forbid the use of both fat tires and fenders. (Long reach sidepulls can allow one, but not usually both). Fenders are vital for riding in the rain and staying dry, and the various wide tires available are useful for shock absorbtion and/or better off-road traction. These are useful options for any cyclist to have.
Also, loaded touring bikes featuring disk brakes are starting to appear. Disc brakes are heavier and signifacntly more powerful than rim brakes. They are a product of the booming downhill mountain bike scene. The reason they're starting to appear arguably has more to do with their cool factor than their practicality, but time will tell.
Gearing choices: A bike's gearing is often the bottom-line determinant of what it can be used for. Cyclists need low gears to be able to climb hills, especially if they're carrying weight, and especially if they don't want to tire themselves out. All touring bikes (except exotic ones with modern internally geared hubs) have triple cranksets, which through their small inner chainring are the key to having a bike with low gearing. Although gearing preferences vary with terrain, loaded touring bikes often need gearing that on the low end ranges from low to as low as possible. This kind of setup usually uses a "microdrive" crankset from a mountain bike component group, because these are the modern cranksets on which the smallest available inner chainrings fit (22 tooth being the smallest, although most cyclists don't go that low). The various light tourers usually come stock with standard road triple cranksets, which only go down to 30 tooth minimum. A 30 tooth ring is quite a bit larger than desirable for some jobs, and you can't go lower without getting a new crankset. One trouble spot in the current bike touring scene is that several major manufacturer's loaded touring offerings come with road triples, which forces a lot of people to immediately do heavy modifications to their stock drivetrain setup.
Shifters: Touring bikes use all varieties of shifters, although most serious loaded tourists prefer bar-end shifters because they are the most reliable. (The same is true for thumb-shifters on touring bikes with flat bars, although no high-quality thumbshifters are being manufactured currently.) STI shifters have a tendency to fail eventually, sometimes without warning, although many tourists choose to risk it for their convenience.
Handlebars: Traditional loaded touring bikes use the curved drop bars of a road bike. The advantage to these is they give the highest number of distinct hand positions, between 4 and 6, each giving a different comfort/aerodynamicness tradeoff. When properly set up (mainly, when one has a stem that positions them appropriately given preferences), drop bars allow fairly upright positions for low-intensity comfort, and fairly aggressive, dropped ones for when you're going for speed or cutting wind drag.
Mountain bike style flat or riser bars are becoming popular on touring bikes. These seem comfortable, but have the overlooked disadvantage of only giving 1 hand position, 2 with bar-ends, and neither of which are very good in comparison to the other options. This owes a lot to people having a negative image of drop bars due to experiences with them on bikes that weren't set up for them at all, or were set up for racing, and also with people being more familiar with flat bars because of the mountain bike era.
Another option are the various mustache bars, popularized by and usually obtained through Rivendell. Although obscure and an obvious sign of someone who really cares to customize their bike, these are very competitive with drop bars in terms of hand position options, and many find them more comfortable.
It should also be noted here that touring bikes tend to have stems with a high rise to them, usually putting the handlebars at least at the level of the saddle. This is to make the cyclist less hunched over. There is no standard for how aggressive a touring bike's "stock position" should be, though, and it's something that many owners change about their bikes. Some useful stems, namely Technomics, are very tall.
Wheels: The demands placed upon a touring wheelset are very high. There should be at least 36 spokes, with many serious tourists preferring to go into the 40's. (Although this is rare for a stock bike). Touring rim are at least double wall and have eyelets. Butted spokes actually help to make a wheel significantly stronger because their greater elasticity spreads the load upon a wheel out among more spokes at any given time. Many tourists prefer cartridge bearing hubs (and everything else) for touring bikes, but this is rare among current stock bikes. Most touring bikes use the standard 700c road size, although some use standard MTB 26".
Tires: Most loaded touring bikes come stock with wider tires, generally between 32c to 38c. Wider tires ("touring tires" and "hybrid tires" are some arbitrary labels that get thrown around) run at lower pressure and so they absorb a fair amount of road shock compared to skinnier, higher pressure tires. The downside is that they add to rolling resistance (friction), which hurts your efficiency some. Different cyclists have different preferences here, and so many run skinny tires on their touring bikes. Some touring tires also have a moderate, non-superficial tread to help off-road. Also, regardless of tire width, most touring cyclists prefer to use tires that have casing and rubber materials which make them flat-resistant and long-lasting.
There are also various types of recumbent (reclined position) bikes used for touring. Most popular are long-wheelbase recumbents for their higher stability, but there are varied opinions about this.
The term "touring bike" also refers to the genre of motorcycle made to carry lots of cargo, go long distances in comfort, etc.