Analogous structures are similar features of different animals that have evolved due to convergent evolution. When two different species live in similar environments, they often evolve in a similar way. This causes the bodies of the two different species to develop similar structures even though they may have started with very different bodies.
Animals with analogous structures are not necessarily very closely related genetically, despite the fact that they may have evolved similar bodily structures. For example, bats, bluebirds and butterflies all use wings to fly. These three creatures, however, are dissimilar in many other aspects, for bats are mammals, bluebirds are birds and butterflies are insects. Likewise, a dolphin and a shark both have flippers, a fin and a similar body shape, but the shark is a fish and the dolphin is a mammal, which makes it more closely related to a rat than to the fish with which it shares the oceans. Because the shark and the dolphin both live in the sea, they have evolved similar bodily structures to cope with life in that particular environment, but they are otherwise very different. This fact has made it difficult, sometimes, for scientists to make accurate classifications of animals, especially before scientists knew how to work with DNA.