It is a tank whose focus, IMO, is on the invertebrate life of a reef, rather than the fish. While people certainly still have fish, the load tends to be less. The emphasis is on the health and growth and structure of corals, anemones, and the various other animals whose lives are often overlooked, yet without their presence, a reef would not exist. In some ways, it is one of the most natural saltwater systems, because it tends to eliminate large cruising fish whose life in the wild can really not be approached in captivity. Rather, a reef tank often focuses on smaller, often territorial fish, as if one took a small section of a reef face and put it into a glass box. Thus, clowns, anthias, basslets, gobies, blennies, dwarf angels, etc become the primary fish, just as they would in the wild if observing a small section of the reef. It is a tank that draws the observer in, to look at the little animals in the cracks and crevices, both day and night, rather than a tank to be observed from a distance, like (IMO) FO or predator tanks. But this is my bias as an invertebrate zoologist.
LR alone does not a reef tank make. Just like sinking a ship does not automatically make a true reef, IMO. While fish are drawn to structure, it is those animals that colonize the surface that make the reef, IMO. Those that grow and in turn support the other animals that live there.