A 'slice' of your HardDisk. (In fact, in some OS's like BSD, partitions are called slices.) Once you have a partition it appears to the OperatingSystem as a different hard disk you have to put a FileSystem on top of it.
In older terms, think of it like breaking your hard disk into a C: and a D: drive. Then, you can do anything you want on your D: drive, even format it, and still have all the information on your C: drive. (Thankfully Linux doesn't use DriveLetters and even Windows is moving away from it.)