We tend to think of homologies in terms of anatomy: the tetrapod limb, insect wings, vertebrate eyes. However, anatomical homologies are just one of many sorts of homologies. Any heritable trait — anything that can be directly or indirectly encoded in DNA — can be a homology:

• A simple trait
  Among milkweed species, having milky sap is a homology.
• A complex structure
  The antennae of beetles and moths are homologous, for example.

• A gene sequence
  The gene that codes for the protein insulin is homologous in humans and cows; both sequences descended from the same ancestral gene sequence.
• A behavior
  Birds and crocodiles both provide parental care to their offspring (as shown below); this behavior was inherited from a common ancestor and is homologous.