Common time in sheet music is represented by a stylized 'C' where the time signature would normally be. It represents 4/4 time, and is so called because it is the most commonly-used time signature.
It looks something like this:
/\
|----| /-------
| |/
|----/---------
| /| / \
|--/-|/\--|----
| | | | \_/
|-| -|--|------
| \ | |
|---\|_/-------
|
\|