Hard disks can be divided into one or more logical disks called partitions. This division is described in the partition table found in sector 0 of the disk.
Linux needs at least one partition, namely for its root file system. It can use swap files and/or swap partitions, but the latter are more efficient. So, usually one will want a second Linux partition dedicated as a swap partition. On Intel compatible hardware, the BIOS that boots the system can often only access the first 1024 cylinders of the disk. For this reason people with large disks often create a third partition, just a few MB large, typically mounted on /boot, to store the kernel image and a few auxiliary files needed at boot time, so as to make sure that this stuff is accessible to the BIOS. There may be reasons of security, ease of administration and backup, or testing, to use more than the minimum number of partitions.
See also: cfdisk