Homology describes the similarity between a human arm, a bat wing, and a whale flipper because they share ancestry.
These forelimbs contain the same basic skeletal pattern: the humerus, radius, ulna, wrist bones, and digits. Their shapes and proportions differ because natural selection modified the ancestral limb for grasping, flying, swimming, and other functions.
Homology is evidence of common descent. The similarity does not mean that all homologous structures perform the same job; it means their underlying developmental or structural origin is historically related.
A common mix-up is homology versus analogy. Analogous structures can perform similar functions while evolving independently, such as the wings of birds and insects. In modern biology, homology can also refer to corresponding genes, DNA sequences, or developmental pathways shared because of inheritance from a common ancestor.