A time signature in music notation is drawn on the staff immediately after the clef. But while the clef is drawn on every staff, the time signature is only drawn on the first staff or grand staff, until the composer changes it during the course of the piece.
Time signature is used to indicate how the beats in the music are divided between measures. It consists of two numbers, one atop the other. The top number indicates how many beats there are per measure. The bottom number indicates what type of note receives a single beat -- 1 means a whole note, 2 a half note, 4 a quarter note, 8 an eighth note, 16 a sixteenth note. For instance, 3/4 would mean three quarter notes per measure (a nice waltz), 2/2 would mean two half notes per measure, and 7/8 would mean seven eighth notes per measure.
A time signature on the grand staff looks something like this, although in proper sheet music the top number spans the top two spaces in the staff and the bottom number spans the bottom two spaces:
/\
/|----| /-------
| | |/
| |----/---------
| | /| 3
| |--/-|/\-------
| | | | | 4
| |-| -|--|------
| | \ | |
| |---\|_/-------
| | |
/ | \|
\ |
| |
| |--------------
| | / \ *
| |-*-----|------
| | | * 3
| |------/-------
| | / 4
| |----/ --------
| | /
\|--------------
Two time signatures are so common they have special symbols. These are called common time (4/4 time) and cut time (2/2 time).