While a whale's fins are similar in structure and function to those of a fish, whales and fish are not closely related, so their similarities are defined as analogous, not homologous. The development of fins in two such unrelated animals is an example of convergent evolution.
In contrast, homologous structures are usually the result of divergent evolution, when two species develop from a common ancestor. A monkey's tail and the human tail bone, or coccyx, are examples of homologous structures. Humans and monkeys developed from a common ancestor that had a tail, but while monkeys kept the structure as they evolved, humans did not.