Features of organisms almost always evolve from pre-existing features of their ancestors; they do not arrive de novo.
In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying structure of bones and muscles. (source: Wikipedia).
Related organisms have homologous characters, which have been inherited (and sometime modified) from an equivalent organ in the common ancestor.
A character may be homologous among species (e.g. 'toes'), but a given character state may not be (e.g., a certain number of toes).
A character (or character state) is defined as homologous in two species if it has been derived from their common ancestor. Determining whether or not characters of two species are homologous can be difficult though! Criteria are: correspondence of position and of structure (not correspondence of function or of shape!). The most important criterium however is to see if its distribution on a phylogenetic tree (based on other characters) indicates continuity of inherentence from their common ancestor.