Borrowed from wikipedia:
If an instrument has a range that is too high or too low for its music to be easily written on bass or treble clef, the music may be written either an octave higher or an octave lower than it sounds, in order to reduce the use of ledger lines. Instruments that "transpose at the octave" are not playing in a different key from concert pitch instruments, but sound an octave higher or lower than written. Some instruments with extremely high or low ranges use a two-octave transposition.
Music for the contrabassoon and the double bass is written on the bass clef, one octave higher than concert pitch. Music for the guitar and, frequently, the tenor voice is written on the treble clef, one octave higher than concert pitch. Music for the piccolo is written on the treble clef, one octave lower than concert pitch. If these instruments did not transpose at the octave many of their pitches would be written far above or below the staff, making reading comparatively cumbersome