The double bass (aka Contrabass, aka doghouse bass, aka standup bass, aka string bass, aka acoustic bass)

Size
There are different sizes of double bass, ranging most commonly from the 1/2 size to the full scale size. 3/4 being the most common. Generally volume is proportional to the size of the bass, and thus full scale basses are the most sought after. Also, the larger the bass you play, it seems the more respect you get. This is probably half due to a macho element in the world of double bass players, but full scale instruments tend to be a bit harder to play, yet usually yield a greater sound. So someone who can skillfully play a full sized instrument is genuinely deserving of additional respect. However, in general most instruments found tend to be of the 3/4 variety. There are well suited for an average sized man. Smaller sizes also exist and are usually used by children as beginner instruments or by shorter players. The smallest that is in any numbers is the 1/2 size. I'd estimate these to be good for 5'4" sized people.

Strings
There are a number of different kinds of strings available to the modern double bassist. I outline here the main types. In general they are mostly made of metal, (usually nickel) and are flat wound. Flat wound strings have a core (of one sort or another) which is wrapped with a flat strip of metal to give a smooth surface for the fingers or bow to work on.

-Orchestra Strings
Designed for orchestra use, these strings are almost exclusively made of metal, and always have flat windings. They need the flat windings so you can play arco on them (see below). These strings are usually of the highest tension of strings available and therefore the loudest. (side note, I prefer Thomastik Spirocores myself)

-Solo Strings
Solo bass strings are like the Orchestra strings but are basically less tightly strung, thus making them a bit less sharp sounding and easier to play with pizzicato with. Popular with jazz players, though they are still ok for orchestral use. Hybrid strings are also available which are good for both and are often used by players who need one bass to serve double duty in both jazz and orchestra.

-Gut Strings
Gut strings are how double bass strings where traditionally made. Gut is lighter than both solo and orchestra type strings. They are light brown and semi-translucent. These are good for players playing authentic chamber music (many of chamber musicians tune down to the older tunings and use older styles of strings for a more authentic sound). Also, many folk,country and rockabilly players use them to get a vintage sound. Gut strings tend to go out of tune fast, which makes them a bit of a hassle. However, they feel nice on your fingers and have a good mellow sound, but are very expensive.

-Nylon Strings
Nylon is basically a cheaper stronger replacement for gut. They stay in tune a little better but don't sound half as good. They tend to be bright white nylon and look a bit gaudy.

-String Pricing
Double bass strings are much more expensive than guitar, electric bass, or violin strings. A four-string set generally costs about 100 USD. Gut usually goes for 200 and I've seen nylon for as low as 50. Used strings are another option, a lot of instrument shops will sell used strings for $30 a set. I prefer this because used orchestra strings are mellow but still have the high tension feeling to them.

Longevity
Luckily to offset the high price, double bass strings last a long time. Most people get at least a year out of a set of orchestra strings, sometimes up to 4 years depending on preference and how much you play.

Techniques

-Pizzicato
This is when you strike the string with your fingers. Usually with the thumb-side of your index finger, The best way to get a good sound is to use as much of the meat on the side of your index finger as possible (right hand). Usually there is no noise of the string hitting the finger board, but if you're playing really hard there can be.

-Arco
Also known as bowing, this is when you take a bow and draw it across a string to make sound. See below...

-Slap Bass
This technique is alleged to have originated around the turn of the century in new orleans. Slap is when you pluck the strings by pulling the string out away from the bass and letting it slap back against the finger board giving a sharp cracking noise when the string strikes the finger board. This adds an audible click to every note played. This is very good if there is no drummer present and can sound good even when there is one around. This is used often in country, rockabilly and blues. Some notable players for this style are Marshall Lytle (of Bill Haley and the Comets) and Willie Dixon (famous for playing with many great bluesmen , writing countless standard and producing for Chess Records).

Musical types
The double bass is used in many musical styles throughout history and the world. Here are some of the major ones.

-Orchestral
In most orchestras there is a double bass section, usually sitting behind the cello's on the conductors right side. Often they sit on high stools to play as orchestral pieces can be rather long. This usually involves arco playing and occasionally pizzicato.

-Jazz
Double bass is key in almost all older jazz (replaced only occasionally by organ or tuba) and in newer jazz (often replaced with electric bass). Some great players to check out include Ray Brown and Charles Mingus.

-Country Music & Rockabilly
Country and Rockabilly bass usually is synonymous with slap bass, though pizzicato playing is also prevalent. Rockabilly is also known for particularly animated bass players (they are known for climbing on, surfing on, spinning and throwing their basses). Some famous rockabilly bass players: Marshall Lytle (Bill Haley and the Comets), Bill Black (Elvis Presley) and Lee Rocker (Stray Cats).