A partition can be thought of as a division or "part" of a real hard disk drive.
A partition is really only a logical separation from the whole drive, but it appears as though the division creates multiple physical drives.
Some terms you'll see associated with a partition include primary, active, extended, and logical partitions. More on this below.
Partitions are also sometimes called disk partitions and when someone uses the word drive, they usually mean a partition with a drive letter assigned.