The tenor clef is one of the C clefs. It looks like a capital K and is positioned so that the point where the three lines meet is on the line that will be used for C4. From this, you can work out which lines are used for the other lines. Historically, there were quite a few C clefs but only the tenor and alto have survived to contemporary use. The alto clef is placed on the third line (middle line) and is also called the viola clef. The tenor (also the cello clef) is placed on the fourth line. One issue that causes some people confusion is that the higher up the staff the clef is placed, the lower the range of the staff. E.G. a C clef on the top line gives the staff a range of B2 to C4 whereas a C clef on the bottom line gives a range of C4 to D5. These days, only the F and G clefs are in common use, and only when placed on the fourth line (F clef) and second line (G clef). These are known as the bass and treble clefs respectively.