Most music these days is written in either bass clef or treble clef, but some music is written in a Cclef. The C clef is moveable: whatever line it centers on is a middle C (pg 120).
The bass and treble clefs were also once moveable, but it is now very rare to see them anywhere but in their standard positions. If you do see a treble or bass clef symbol in an unusual place, remember: treble clef is a G clef; its spiral curls around a G. Bass clef is an F clef; its two dots center around an F.
Much more common is the use of a treble clef that is meant to be read one octave below the written pitch. Since many people are uncomfortable reading bass clef, someone writing music that is meant to sound in the region of the bass clef may decide to write it in the treble clef so that it is easy to read. A very small "8" at the bottom of the treble clef symbol means that the notes should sound one octave lower than they are written.