The sirloin is beef steak cut from the lower portion of the ribs, continuing off the tenderloin from which filet mignon is cut. Of the steaks typically considered to be premium steaks, the sirloin is the cheapest, because the muscles still do quite a bit of work.
The sirloin is actually divided into several types of steak. The top sirloin is the most prized of these. The bottom sirloin is less tender, much larger, and is typically what is offered when one just buys sirloin steaks instead of steaks specifically marked top sirloin. The bottom sirloin in turn connects to the sirloin tip roast, which is generally considered to be a good, if somewhat tough, roast.