This is computer-speak for the dimensions of the drive. New technology has allowed smaller drives, in physical size, to be produced bringing down the price as capacity increases. Unfortunately for the novice computer buyer, hard drives are not all alike. They vary in many ways, including ease of installation, form factor, performance and price. There's also a difference between IDE and SCSI drives.
SCSI drives have a reputation for being faster, and more expensive.
They do also require a special SCSI controller card, an added expense. Most PCs have IDE drives and in typical applications there is no significant performance difference between these and SCSI drives. The SCSI really comes into its own with file servers, so an IDE drive is suitable for most business and home users. One of the best things about upgrading your hard drive is that you don't actually replace your existing hard drive. Instead you add a slave' to your existing 'master' drive, allowing the two drives to work in conjunction.