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.)

If you have one hard disk and want a DualBooting system, you will need to arrange your hard disk into two or more partitions. See PartitioningSuggestions.

In Linux, partitions are managed with fdisk(8) or one of the newer utils like cfdisk(8).

If you have a volume larger than about 2 terabytes, you need to use GPT rather than the normal IBM PC / MSDOS partition table.