The hard drive in your PC is most likely one big object. It's the simplest way to look a hard drive -- like it's one big filing drawer full of folders and files. But you can, with a little work, split that hard drive into many different file drawers -- one for system files, one for your documents, one for music and so on.
Partitioning a hard drive is helpful for both organization and speed. It also allows you to, for example, run Windows and Linux together on the same PC or, using Apple's BootCamp, run Windows and Mac OS X alongside each other.