A drummer is a musician who plays the drums. The drummer in a rock band might play a huge set of many different drums and other percussion instruments.
A professional drummer could play in a marching band, a classical symphony, or a country and western group. A casual drummer can be someone who taps on the bongoes in a city park or takes drumming lessons and practices in her basement. The word drummer comes from drum, which has a Germanic, imitative root — it's a word that sounds like its meaning.