A language family is group of languages related through descent from a common ancestor, called the proto-language of that family. The term 'family' comes from the tree model of language origination in historical linguistics, which makes use of a metaphor comparing languages to people in a biological family tree or in a subsequent modification to species in a phylogenetic tree of evolutionary taxonomy. All the apparently biological terms are used only in the metaphoric sense: No actual biology relationship is implied by the metaphor.

As of early 2009, SIL Ethnologue catalogued 6,909 living human languages. A "living language" is simply one that is in wide use as a primary form of communication by a specific group of living people. The exact number of known living languages will vary from 5,000 to 10,000, depending generally on the precision of one's definition of "language", and in particular on how one classifies dialects. There are also many dead and extinct languages.