A partition in personal computers is a logical division of a hard disk.
This is typically created so that a user can have a different operating system on the same hard disk, or to create the appearance of having a separate hard drive.
Having the appearance of a separate hard drive can be useful for things like file management or for multiple users on the same computer.
Partitions can be created when the user formats the hard disk, the partitions are usually labelled the C: drive, A: and so on. These are typically reserved for diskette drives.
