There are more than 70 types of stainless steel standard wrought alloys, with many more specialized compositions.
304 - the afore mentioned most common 18-8 type stainless. An austenitic (non magnetic) type of stainless.
302 - similar to 304, but with slightly higher carbon content, not as easy to weld as 304 as a result. Considered the general purpose alloy of the 300 austenitic series.
316 - 18-10 stainless, with molybdenum, more corrosion and creep resistant than 304
316L - the L stands for low carbon, better for welding than 316
430 - generally 18% chromium, no nickel, with manganese, silicon, phosphorus, sulfur. Known as wrought ferritic stainless general purpose alloy. Magnetic and less ductile than the austenitics.
These previous alloys cannot be hardened appreciably by heat treatment. This brings up the subject of "martenistic" stainless, which can be be heat hardened. Thus they can be stamped out and machined when relatively soft, and then heat hardened and ground after becoming quite hard. High quality knives often will be type 440. The 440 series contains 16 to 18% chromium, no nickel, plus varying amounts of molydenum, manganese, silicon, and between .6 and 1.2% carbon. 440C is a premium alloy with high (1.2%) carbon. It's also very expensive.
Here's a Wikipedia article on stainless that looks to be fairly accurate:
http://en.wikipedia.org/wiki/Stainless_steel