Take a look at your camera, and there’s a good chance it’ll have this symbol: Φ — a small circle bisected by a long line that looks like a hieroglyph of Saturn. If you’ve always wondered what it means, today’s your lucky day: it’s called a “film plane mark” (or “focal plane mark”, depending on who you ask), and indicates exactly where the film (or sensor) plane is inside the camera body. One reason the mark is useful is that macro photographers often want to determine the exact distance between their subject and the film plane, and the mark can make this calculation much easier.
P.S. For a complete list of common camera symbols, check out this page on PhotoNotes.org.