The alto clef is used in sheet music for the viola and other instruments whose range dwells somewhere between the treble clef and the bass clef.
It's also called the C clef because the center line is middle C. Musically speaking, if you combined the bottom two lines of the treble clef and the top two of the bass clef, and added a fifth line between them, you'd have the alto clef.
In music notation, the alto clef looks something like this:
II |---------
II | * \
II |------|--
II | |__/
II |-{ __----
II | | \
II |------|--
II | *__/
II |---------