It is well established that certain structures are homologous. These include things like the arm of a human and the flipper of a seal and the eing of a bird. They are constructed from the same basic elements arranged in a similar manner. It is also well established that molecules can be homologous. Thus, the globin proteins or genes of humans are homologous to those of a horse or a bat.
Homology indicates a similarity established through common descent.
Our forearms are homologous to those of birds because they arose from a common ancestor. A fly hair and a vertebrate hair would be examples of analogy. The wing of a bat and the wing of a bird would be examples of convergent structures. There is now evidence that certain developmental pathways are themselves homologous both within an organism and between phyla. The molecular homology of the Pax6 and other genes mixes function and structure, and neither expression pattern nor structure is a good criterion for homology.