Cantilever brakes are good for fat or semi-fat knobby tires, or fenders-all things which don't fit under the arch of sidepulls. And, cantilevers are more powerful than single-pivot sidepulls, too. Power and good clearance--that's why people like them.
Frames designed for cantilever brakes are more trouble (and therefore costly) to build than are frames for sidepull brakes. The builder has to braze on a rear cable housing stop, and four cantilever bosses to mount the brakes themselves, and they have to go in just the right spot.
The mechanic who sets cantilevers has extra cables to deal with, because unlike sidepulls, which have a direct connection to the brake lever via a single cable and some cable housing, cantilevers have a center cable (equivalent to the sidepull's only cable) which connects to a "crossover cable" or "straddle wire" by means of a straddle wire hanger.
There are two main styles of cantilevers: Traditional, sometimes called "high profile" cantilevers; and Low Profile cantilevers.
High profile cantilevers stick out to the side more, and some people whine that they hit their heels on them, or they get gouged by them during cyclo-cross dismounts; and they can make it harder to mount panniers on touring bikes. But none of these drawbacks is a big deal, or a constant or inevitable problem, and fans of this high profile style like their looks and the positive feel they have that the lower profile models (below) don't.
Low Profile cantilevers don't stick out as far, offer better heel and body clearance, and can work better with panniers, depending on particulars we won't go into here. Mostly, though, they came about because mountain bikes tend to have small head tubes, which require lower straddle wires than you get with high profile cantilevers.
Either cantilever style can and have been forged, cast, machined, and stamped.