You can buy a hard drive as just a drive (and put it into your desktop), or you can buy an external harddrive, which is a regular hard drive that is mounted in a little box, also called enclosure, and has an internal connector to the hard drive that converts the regular drive connection to a USB or Firewire connector.
Examples would be:
External drives:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822101121
http://www.newegg.com/Product/Product.aspx?Item=N82E16822186152
http://www.newegg.com/Product/Product.aspx?Item=N82E16822154294
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136321
Hard drive that needs to get mounted in an enclosure:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148373
You have to make sure, that the drive is properly working for you operating system (Windows or Mac), and that the connector is compatible with your computer (USB, USB2, Firewire400/800).
