Groupings of languages which can all be traced back to a common ancestor language they evolved from, for instance the Romance languages are all descended from Latin.
Some well-known language families are Algonquian, Athabaskan, Austronesian, Bantu, Cushitic, Dravidian, Indo-European, Mayan, Otomanguean, Pama-Nyungan, Salishan, Semitic, Sino-Tibetan, Siouan, Tai, Tupian, Turkic, Uralic and Uto-Aztecan. There are some 300 independent language families (or genetic units, which include both families of languages and language isolates) in the world.
Language families can be of different magnitudes, so that some larger-scale families may include smaller-scale families among their subgroups or branches. The term ‘subgroup’ (also subfamily, branch) refers to a group of languages within a language family that are more closely related to each other than to other languages of that family – a subgroup is a branch of a family. As a proto-language (for example, Proto-Indo-European) diversifies, it develops daughter languages (such as Proto-Germanic, Proto-Celtic); if a daughter (say Proto-Germanic) subsequently splits up itself and develops daughter languages of its own (English, German etc.) they then constitute members of a subgroup (the Germanic languages), and the original daughter language (Proto-Germanic) becomes an intermediate proto-language, occupying the roles of both parent and descendant. Germanic, while a language family in its own right, is at the same time a subfamily (subgroup) of the broader, higher-order Indo-European family of which it is a branch.
Language family names often end in the suffix –an (as for example, Indo-European, Dravidian etc.). Some end in -ic (as in Cushitic, Turkic, Uralic [called Uralian by some] etc.) Such suffixes are helpful, since some language families are named for a prominent language in the family and without the suffix it would be difficult to distinguish the whole family from that single member language; for example, Mayan is a family of some thirty-one languages, while Maya (or Yucatec Maya) is one of the languages of this family; Tucano is a single language, and Tucanoan is the family to which Tucano belongs.
There are a number of terms for postulated but unproven, more inclusive hypothesized language families, proposed distant genetic relationships): stock, phylum and the compounding element ‘macro-’ (as in Macro-Penutian, Macro-Siouan etc.). The proposed distant genetic relationships are much debated. Some scholars employ ‘stock’ or ‘phylum’ in the sense of a language family that is large enough to include well-defined or older subfamilies; however, this usage is often confused with the more common employment of these terms for undemonstrated hypotheses of remote linguistic affiliation, making the use of these terms controversial and often confusing. See also classification of languages, macro-family, subgrouping.
Source: A Glossary of Historical Linguistics.