The sirloin steak is cut from the lower portion of the ribs, it continues off the tenderloin where filet mignon is cut from. Sirloin is considered one of the cheapest premium steaks, because the muscles still do quite a bit of work making the meat tough.
The sirloin is actually divided into a few types of steak.
The top sirloin is the most prized of these. The bottom sirloin is a bit less tender, and is a much larger piece of meat. This is typically what is offered when you go to the store and see a steak marked Sirloin. Make sure that what you buy is specifically marked, if your not sure ask your butcher. The bottom sirloin connects to the sirloin tip roast, which is generally considered to be a good, semi tough, roast.
Sirloin is definitely a steak for the grill, just be sure to marinate to tenderize and add some flavor.