Disk partitioning is the act of dividing a hard drive or other storage media into seperate partitions, which are sections of a single physical device but behaving logically as separate and unique addressable storage devices. The BIOS of the computer is informed of the partitioning through a special area near the beginning of the hard drive called the partition table.
From an end user's point of view, partitioning is important as an initial step to installing a Linux system. Most importantly it can be used to create dual boot systems which allow to run different operation systems on the same computer. For example, a simple dual-boot partitioning scheme might have an NTFS partition containing Windows XP, a partition for a Linux root directory, a third partition for home directories, and a Linux swap partition.
Each physical storage device (e.g. disk drive) requires separate partitioning. BIOS and operating system attempt to collect all partitioning information at system startup and create a coherent device reference table for the user.
Having separate partitions for /home, /var, /usr and /tmp can have some advantages over one single (large) partition. Having /var on a separate partition prevents accidental log-files or mail directories to fill the entire filesystem.
Careful determination of the sizes of each partition is important, but difficult. Changing partition allocation afterwards is cumbersome unless LVM is used.