A rock band is a musical group which plays rock and roll, rock, and pop music. It is composed of three or more members (usually four or five) who play different instruments.
Rock bands usually have a percussion element, which is often the drums. They also have at least one electric guitar player, a vocalist, and either an electric bassist or a pianist (or both).
Rock bands play songs. Generally, they perform prepared sets of originally-written songs or, if they are a cover band, play cover songs in concerts.