What is the difference between the alto and tenor clefs?

For instruments that use a lower register, using the bass clef would sometimes mean lots of ledger lines, which can make a piece of music hard to read.  So, the alto and tenor clefs are used.  The only difference between these two 'C' clefs is that they indicate a different line for middle C to be drawn on.  The alto clef places it in the middle of the stave; the tenor clef places it on the second line from the top.  This allows them to be used for different instruments with different registers.