A partition is a physical subdivision of the space on a hard-disk.
A standard hard-drive can have up to four primary partitions, and within the fourth partition additional extended partitions.
In the Linux environment, programs such as parted are used to manage the partition table. In MS/DOS, the program is named parted.
Merely defining a partition does not put anything into it! You must format the partition, using Linux programs such as mkfs or mkswap or their Windows equivalents, to make them usable to an operating-system.
Each entry in the partition table contains a partition-type code which enables an operating-system to anticipate what the partition actually contains, and to decide whether or not to ignore the partition completely.
Warning: As you may have guessed, fiddling with the partitions on a hard-disk can result in the complete loss of information on the drive!
It is strongly recommended that you print or write-down a copy of the partition layout on your drive(s) and to put that information in a safe place, such as a manila envelope taped to the side of the machine. If the partition-table is damaged for any reason, this will greatly simplify the task of reconstructing it.