One of many percussion instruments, I think they're made of bronze. They range in diameter from finger cymbals to huge 40" ride cymbals, and have a range of thicknesses so that you can produce a variety of different sounds depending on how you play them or what mallets you use on them. Some of them have rivets in them that produce a sizzle effect. Arguably the best cymbal manufacturer is Zildjan.
Drumspeak words that describe cymbal sounds include "pshhh", "psht", "chee", "chu-pshh", "ting", and "dink".