You find a large curved piece of steel with several 3/4" holes in it but you have no idea what "it" is. There are five categories the image may go into to; Plant, Animal, Mineral, Man-Made, and No Clue. "it" would go into the "Man-Made" category. Other users (and eventually recognition software) would begin to offer their ideas on what "it" may be. So when a user says "That looks like a 1943 McCormick 2-row plow.", you could then immediately search the term "1943 McCormick 2-row plow".