Briefly, a copyright protects the work of creators of literary, dramatic, musical, and artistic works. Copyright applies to the form of the expression, not the substance, so the written instructions for a computer (i.e., the lines of progam code) are copyrightable, but what the instructions do is not.
Copyright is clearly more complicated than this, though. For a better understanding, you may wish to consult some of the following copyright information on the WWW: