Drums are a subset of percussion instruments played by beating a "head" stretched over a hollow "shell."
A percussion group of musical instruments.
Drums consist of at least one membrane or surface (metal in the case of steel drums), called a drumhead or drum skin, that is stretched over a shell and struck, either directly with the player's hands, or with a drumstick, to produce sound.
There is often a "resonance head" on the underside of the drum, these are usually tuned to a slightly lower pitch than the top drumhead.
(mostly from wikipedia)