Two major concepts of comparative anatomy:
1. Homologous structures are structures (body parts/anatomy) which are similar in different species because the species have a common ancestor. They may not perform the same function. The classic example of this is the forelimb structure of cats and whales.
2. Analogous structures are structures which are similar in different oragnisms because they evolved in a similar environment and do not have a common ancestor. They usually serve the same purpose or similar purposes. An example is the torpedo body shape of porpoises and sharks. They both evolved in a water environment, but have different ancestors.