The classification of natural languages can be performed on the basis of different underlying principles (different closeness notions, respecting different properties and relations between languages); important directions of present classifications are:
* paying attention to the historical evolution of languages results in a genetic classification of languages—which is based on genetic relatedness of languages,
* paying attention to the internal structure of languages (grammar) results in a typological classification of languages—which is based on similarity of one or more components of the language’s grammar across languages,
* and respecting geographical closeness and contacts between language-speaking communities results in areal groupings of languages.
The different classifications do not match each other and are not expected to, but the correlation between them is an important point for many linguistic research works. (There is a parallel to the classification of species in biological phylogenetics here: consider monophyletic vs. polyphyletic groups of species.)
The task of genetic classification belongs to the field of historical-comparative linguistics, of typological—to linguistic typology.