Typically, this is a metallic image of a fish, flat and five inches long, which has legs and the word Darwin
inside of it. A Darwin Fish
may also be that same image
even if not cast in metal - for instance, if the same image was drawn on a piece of paper, many would call this a Darwin Fish
rather than merely a picture of one.
Most often, a Darwin Fish is affixed to the back of a car, in much the same way that a Christian might affix that ancient symbol of Chistianity, the image of a fish. The modern version of this latter symbol has the word Jesus inside it. The most ancient version of it did not, since it was often used to communicate the presence of Chrisitans to other Christians while minimizing the danger of alerting persecutors to their presence.
The Darwin Fish is a symbol of evolution, with the legs in particular symbolizing the continuity between fish and vertebrates as discussed by Darwin. The resemblance to the Christian symbol is of course no accident. Oddly enough, some who use the Darwin Fish as a symbol consider it a link between Christianity and evolution and a symbol of the fact they can be reconciled, while others (both people who use and people who hate this symbol) consider it a mockery of Christianity.
As with many ideas, there is more than one story current about the origin of the Darwin Fish.