Three-dimensional art created by looping, bending, and twisting wire. It's not as involved or delicate as traditional sculpture -- mostly because you don't have to buy a new block of marble or smash your clay face in if you screw up once.
As far as tools required, all you need to start off are wire cutters and a spool of the stuff itself. While it's possible to flex the wire until it breaks, it's murder on your fingers. Needle nose pliers are optional, but also recommended. And if you don't like the insulation, get a wire stripper or something else to take it off (or use uncoated wire).
The wire itself has the greatest role in determining what you can get away with (the other factors are your dexterity and whether or not you got the pliers like I told you). Spiral-bound notebooks have very stiff wire; it can be used, but not easily. I use ordinary copper wire from RadioShack, which is very flexible but still holds its shape decently.
Now, actually making the thing. I assume you have some idea what you want out of it.
The easiest method basically involves using the wire to define the lines of the object. Polygonal modeling outside a computer! Unless you love triangles (and are very patient), though, the shapes will need some kind of support in the form of more wire running through diagonals.
If it doesn't look quite good enough, you can wrap yet more wire around it to give a solid shape. It consumes lots of the stuff, though, so you have been warned. It's probably possible to put a sculpture together using only spring-shapes and spirals, but I haven't tried it. Too messy for me (plus, I think it'd be at least ten times as difficult -- especially with my preferred type of wire).
The other method I know of is to create shapes (say, circles) and link them together to form a whole. I haven't tried it, but one of the art professors at the local community college has a two-foot-high clown done almost entirely like this standing outside her office. Everything except his hat and the balls he's juggling is interlinked circles. This is one of the reasons she's teaching, and I'm learning.
The medium is best suited to organic or at least partially organic forms. While the five-inch BattleMech may protest otherwise, it was a lot of trouble -- and many of the wires in it are still curved. The wyvern was much easier -- and, an added benefit from the wire, it's realistically poseable (well, at least as realistically as you can get for a creature of fantasy).
So far, the most complicated piece I've finished is a heavily modified fabricant, a biomechanical creature from the game Morrowind. It took forever and a day and, I think, a full spool in spite of its small size (I wrapped it to make it solid). But hey, it balances on two legs...