A crab pot is a trap designed for catching crabs. That is - the delicious, ocean-dwelling ones, of course. Pervert.
In its simplest form, a crab pot is just a cylinder of netting over a metal framework, with a couple of one-way doors, a container for bait, and a long line with a float on top. This type is three or four feet across, and weigh about twenty pounds. Commercial pots are usually all-metal and somewhat larger. There are also much fancier consumer traps, that have more elaborate doors, fancier bait cages, and gimmicks like flashing lights (that are supposed to attract crabs.) I've only seen one of these once, when a neighbor of ours was deploying it - that is, throwing it (line, float, and all) over the side of his boat. The water was a lot deeper than he thought, and the entire assembly rapidly disappeared into the murky depths, lights flashing sadly all the way down. Nobody ever saw it again.
Anyhoo, the process of crab trapping is about as simple as crabs themselves. First, you put a bunch of seafood tripe - fish heads and guts, that sort of thing, the messier and juicier the better - into the bait container. Many people have secret bait recipes they won't share with anyone; my family likes to use bulk turkey legs, but that's mostly because we can't fish well enough to catch the aforementioned tripe. Once the bait container is loaded with crab snacks, you find a place where you're sure there are lots of crabs - or try to worm someone else's Secret Crabbing Spot out of them, which is only a little harder than getting their Secret Bait Recipe. Then you throw your pot over the side (holding onto the line - see above.) Assuming the trap hits bottom (30 or 40 feet is good) before you run out of line - and make sure you've got a little extra to compensate for the tides - your part is done. You go away for a while, and wait.
While you're waiting, the crabs smell the bait and climb all over the trap; they're clever enough to find the one-way doors and get in, but not clever enough to monkey the doors and get out. After a day or two, you return, pull up your crab pot, and extract the crabs (most of which will be female, or too small. All of them will want to pinch your fingers off, so it's a good idea to grab them from behind, and to wear a good sturdy glove just in case.) This extraction is accomplished via a special "human door" in the pot - usually the top or an entire side is hinged, which allows you and your opposable thumbs (which can untie the string that holds the human door closed) easy access to the tasty but irritable contents of the trap. By state law (in Washington, at least) the human door now must be tied shut with biodegradable string; this prevents pots that are not recoverable from becoming permanent death traps. Even if this isn't law in your area, it's still good policy.
One final note - if you're anything like me, you'll often find something interesting and/or horrible (besides crabs) in your crab pot. I once hauled up a sun star (a starfish with 50 or 60 arms,) which had spread itself over the pot and ejected its stomach into it in an attempt to get at the bait. Ick.
One more final note - don't cook crabs by throwing them straight into boiling water, as seems to be most common. Instead, put them in the freezer for a half hour first to anaesthetize them. Also, don't 'drown' them in fresh water - it's even worse than boiling.