Clefs
Music is a mixture of rhythm & pitch. Music notation shows the duration of the note and the pitch of the note.
The notes in the higher range are notated on a staff with a treble clef.
The notes in the lower range are notated on a staff with a bass clef.
A drum clef is used to notate rhythmic patterns for a drum kit and percussion.
The viola uses an alto clef. This is because the range of the instrument would require the use of a lot of ledger lines beneath the staff with a treble clef or above the staff with a bass clef.
With an alto clef, the middle line of the staff becomes middle C.
With a tenor clef, the fourth line of the staff becomes middle C.
The bassoon uses the tenor clef for the same reason as the viola uses an alto clef, to avoid lots of ledger lines due to its range.