Throughout the centuries there have been many music clefs. We are concerned with the five (5) clefs seen most often in music of the modern period. These are the Treble Clef, Alto Clef, Tenor Clef and Bass Clef. The 5th clef is for instruments that don't produce defined pitches, such as drums and percussion.
Clefs are used to tell the musician what notes to play and in what octave. Without a variety of clefs, music would have to be written using lots of ledger lines (those telephone pole looking things). Clefs allow the music to sit mostly within the "staff" (the 5 lines that cross the page).
Each clef is shown with "Middle C" (The middle "C" on a piano keyboard)
The Treble Clef (also known as the "G" clef) and the Bass Clef (also known as the "F" clef) are probably familiar to you. By far, these are the most commonly used clefs. Most instruments today use one or both of these clefs exclusively or at least primarily.
The other two clefs might look a bit strange if you have not seen them before. Both the Alto Clef and the Tenor Clef are known as "movable" clefs. Notice how they both use the same shape, but the clef appears in a different position on the staff. Notice how "Middle C" moves with the clefs as well, and how it appears where the two semi-circles come together. The Viola's primary clef is the Alto Clef. The Tenor Clef is used by certain instruments as they move into their upper registers to avoid overuse of ledger lines. Some instruments that might use the Tenor Clef are Cellos, Trombones and Bassoons.
The Percussion Clef: Clef used for instruments that don't produce defined pitches, such as drums or percussion.