Homologous structures have a common ancestor, they have the same basic structure, but have been altered by evolution to fit the needs of the organism. 
For example in vertebrates, the forelimbs all have the same basic structure, with bone, muscle, ligament, tendon and skin, all arranged in a similar format, but altered to fit the organisms needs. a human will have a hand, a bird will have a wing, a seal will have a flipper, and a horse will have a hoof.