As Webster says, a dory is essentially a broad, flat-bottomed, keel-less skiff with flared sides. But...
Why would I want a dory?
Seaworthiness. Dories are stable. Most other sailboat hulls have a rounded cross-section and a large keel, to resist blowback while tacking. If you play with a toy sailboat in the bathtub, you can feel it want to move along the forward/backward axis, and resist turning. That's great if all you want to do is sail, but it can backfire in heavy weather. The huge lateral surface of the keel makes act like an underwater sail: the boat tends to heel (tip) in curents abeam (from the side), and if the current is going against the wind, it could roll right over and capsize.
Dories are much shallower and have no keels, so they ride over waves instead of getting tugged around by them. Their sharply canted sides lift up as a wave hits, and their width makes them resist heeling, as the windward side cantalevers out of the water for balance. This makes dories tippier than other hulls for the first 20° or so, but much more stable by the time there's a risk of capsizing, at about 40°. They're more likely to make you seasick, but less likely to make you dead.
Good handling. The flow of water around a dory hull is relatively simple; there are no odd turbulence effects or other surprises. The sharp bow and rounded shape makes them steady but easy to turn. Still, they are displacement hulls and don't like to go very fast: 20 knots (30kph) is about the limit even for large dories, and smaller ones may be ineffecient over 8 knots (14kph). If you're willing to go that slow, you can do whatever you want. (Argh. I see there is no node on planing versus displacement. The gist of it that some boats are designed to come most of the way out of the water and go very fast, and other boats are designed to keep smooth laminar flow. It's like the difference between a pigeon and an F-22; they're using the air in completely different ways. Dories like to go slow and steady, displacing the water instead of avoiding it.)
Most dories row well, because the freeboard is small and steady — the gunnels come down low amidships, and follow the water. And half the point of a dory is loading: you can cram a truly amazing amount of stuff into them — especially the bow and stern, which form corners — without the steering getting soggy. Many dories are designed to be at their best under several tons of cargo, which is more than most small boats can carry at all. Again, the simplicity of the hull keeps the dynamics nice and linear.
Easy construction and storage. Take the skins of three melon slices, glue them together, and there's your dory. Keels are difficult to make well, but flat bottoms are as easy as floors. The sides can be done by the lapstrake method: bend, overlap, and attach some planks, something like very wide shingles. Some Banks dories were made out of just eleven pieces of wood: four strakes on each side, two planks for the bottom, and one little strip for the transom. Compared to other wooden boats, that's a breeze; working in plywood or fiberglass, a six-year-old could knock them out. The flat hull makes them easy to stack and easy to beach, and thus to maintain: no trailers or drydocks needed, just tides.
Why wouldn't I want a dory?
Probably because you want to go fast. Maybe because you want to sail on a calm day without a large load. Perhaps because you need to do some heavy hauling, when a dory would be inefficient: other hulls are better past about 35 feet (10m), because that's too long to ride over most waves. Where other small boats are touring bikes, dories are mountainbikes.
Kinds of dories
Swampscott dories were developed by Theophilus Bracket in the lobstering town of Swampscott, Massachusetts, around 1840, and were the first really popular model. Their sides are curved rather than straight in cross-section, which makes them easier to sail. They're good for relatively calm, in-shore work. http://www.swampscott.org/dory.htm has a cool model when their site isn't down.
Banks dories were the craft of choice for fishing the Grand Banks. Big sloops carried dozens of these boats out to the Banks stacked up on deck and sent them out one by one to fish; a few days later, they would come back for them, put the fish in the hold, and stack the dories up again like dirty dishes. To hold severals tons of seafood on the open ocean, they had to be safe under heavy loads — nowadays, according to http://www.pearsonunlimited.com/DorySpecs.html, they're used by lifeguards.
St Pierre dories are just big Banks dories with high bows and sterns. They're named after Saint-Pierre and Miquelon, two little nominally-French islands way off in the Atlantic, which have always had terrible weather and until recently had wonderful fishing. The boats are bizarrely sturdy things, with some sort of death-defying utilitarian elegance. At http://bsd.webschooner.com/gnm/shipyard/ the Guild of Nefarious Mariners lays it all down.
Carolina Dories are the least dory-like of the classic dories; they were designed for inboard engines and river-running (though not to the exclusion of sail and open water), so they're relatively low and shallow, with wide transoms. Some models are semi-planing: at about 16 knots (25kph), the bow will rise a little and allow for another few knots.
Sources and interesting dory projects:
- http://rheme.net/dory/ (My own sketchy plans for a Banksish dory, done from memory before I found XI, below.)
- http://www.boat-links.com/DepoeBay/00/Bonus/Dory-3.jpg (Well, that's what a dory looks like.)
- http://www.nationaldorymans.org/ (The National Doryman's Association.)
- http://www.rockisland.com/~kyak/udl.html (An umiak-dory hybrid! Rawk!)
- http://www.greatwoodboats.com/csb18.jpg (Oddly fascinating, isn't it?)
- http://home.att.net/~td2evers/dorylinks.htm (Golly, those are a lot of dory links.)
- http://www.c2boats.com/intros/GBDtanit.htm (Man, look at that thing — is it even touching the water?)
- http://www.duckworksmagazine.com/articles/britishdories/gav.htm (A cute little bit on British dories.)
- http://dragonflycanoe.com/stephens/progress10.html (There's a plan of a classic dory at the bottom.)
- http://museum.gov.ns.ca/dory/ (I quote:
Dories were popular in their day for much the same reason that styrofoam cups are today.)
- http://travelback.com/origami/ (Marginal as a dory, but brilliant as an origami pirate ship.)
This writeup is dedicated to the Puffin, an obsolete 27-foot St Pierre with an inboard diesel, and the Urchin, a 17-foot Carolina which broke loose in a storm and holed itself on the beach.