Similarity between organisms can arise for different evolutionary reasons.

A homologous character is similar because it was inherited from a common ancestor. The character may later be modified for different functions.

An analogous character is similar in function or appearance but evolved independently rather than being inherited in that form from a recent common ancestor. Such independent evolution of similar features is convergent evolution.

For example, the forelimb bones of bats, whales and humans are homologous: the same basic skeletal pattern was inherited from a common tetrapod ancestor and modified for flight, swimming or manipulation.

By contrast, the aerodynamic wing surfaces of birds and insects arose independently. Their similarity reflects similar functional demands, not inheritance of a wing from a recent common ancestor.

This distinction matters for phylogenetic inference. If an analogous similarity is mistakenly treated as shared ancestry, unrelated lineages can be grouped together incorrectly.

Homology is therefore a historical claim, not merely a statement that two structures look alike. Evidence can come from anatomy, development, genomic position and molecular sequence.

A useful question is:

$$\boxed{\text{Is the similarity best explained by inheritance from a common ancestor or by independent evolution?}}$$

Phylogenetic analysis aims to use characters that preserve information about shared ancestry while recognizing that convergence can obscure it.