Partition
To understand what partitions are I will describe it as if I am building a house.
When you decide to build a house, you choose the layout of the land such as the size of the land you want to own and how it is going to be laid out. The hard drive is the entire land. The partition is how the hard drive is going to be setup. Just like land can be divided up to have numerous houses, the hard drive can be divided up to number of partitions. Each piece of land will have its own address and each partition has it's own address. The first partition is given the drive letter of C: Then each partition that is made for the hard drive is given a drive letter. If you have two hard drives then the second drive will take letters after the first drive.
Example 1: You have a 30 gig hard drive on primary and you have a 20 gig hard drive on secondary. You decided you want to divide the 30 gig drive up to 3 10 gig partitions and leave the 20 gig as one big drive. Then the 30 gig drive will be assigned to Drive C, Drive D, and Drive E. The 20 gig drive will be assigned drive F. Now if you have any CD-ROM drives or other types of drives, they will be assigned drive letters after the hard drives.
Example 2: You have a 30 gig hard drive and a 20 gig hard drive. You decide to use the entire 30 gigs for the C drive. That means the 20 gig drive will become drive D. Now if you have any CD-ROM drives or other types of drives, they will be assigned drive letters after the hard drives.
The only exception to this is if you have a zip drive and Windows XP. If you don't disconnect the zip drive before installing Windows XP, the zip drive will become drive C and the hard drive will take the next drive letter after the CD-ROM drives. This will cause a problem and some programs will not install. See Drive letter is wrong for further information.