Homology is the existence of shared ancestry between a pair of structures, or genes, in different species. Evolutionary theory says about the homologus structures and identify that some traits have commom ancestors. The different organisms have similar traits because they are evolved from the common ancestor.
For example: The forelimb of a whale, wing of a bat, leg of cheetah, claw of a mole & hand of a man. These organs externally looks different but internally have same structure.... :)