A partition is a section of a hard disk. When you format a hard disk, you can usually choose the number of partitions you want. The computer will recognize each partition as a separate disk, and each will show up under "My Computer" (Windows) or on the desktop (Macintosh).
So why would you want to create multiple partitions? Well, there are two main reasons. One is if you want to have multiple operating systems on your computer. Typically, an operating system needs to be installed on its own partition so that it won't conflict with other operating systems on the machine. The other reason is that multiple partitions can improve the efficiency of your hard disk. On larger disks, the cluster or block size (which is the minimum amount of space a file can take up), is larger than on small disks. This can result in a waste of disk space if you have a lot of small files. So creating multiple partitions can actually give you more space. Yep, more room for all those MP3s. "Partition" can also be used as a verb, meaning to create multiple partitions.