Everything you keep on your computer is on the hard drive. Not just documents, pictures, and music - your programs, preferences, and even your operating system are all stored on this drive. There are several metal discs inside that spin, and the faster they spin, the more quickly your computer can read and write data to/from it. Solid State Drives (SSD) are significantly faster, as there are no moving parts involved, and can often last longer.
Things to look for: