What can be copyrighted? It's a big question, but the answer can be summed up fairly neatly as "any expression of an idea." It can't be an exact copy of something that already exists, and it can't be the idea itself. Expressions of ideas often fall under the heading of intellectual property—literature, music, architecture, art or photography, for example—so the chances aren't too slim that you're going to want to know the basics about copyright.
Whether you're a professional writer, photographer or graphic designer, or just someone who worries about whether you really own all those pictures of yourself on Facebook, plenty of people have some reason to be interested in copyright law.