The Hard Drive is the primary storage device of your computer to where all of your applications, operating system, and files are stored. The memory size of your Hard Drive will determine how much you can put on it, as well the speed of your hard drive will determine how fast it can transfer data.
When purchasing a desktop hard drive I would simply recommend getting a 7200RPM SATA 3.0Gb/s, 250GB or more hard drive. I purchased the
Western Digital Caviar Blue 640GB 3.5″ SATA 3.0Gb/s Internal Hard Drive -Bare Drive, which so far is enough room for me, but I also have room to add another hard drive later.