Difficulties in describing such notions as vernacular, common slang and slang and the reference of certain lexical units or texts to a particular language phenomenon stem from the variety of opinions and ways of defining each of them. These debatable questions have become more distinctive in Russian linguistics with the growing contacts with European linguistic schools and therefore copying the terms without adapting them to Russian theory of language. The using of these terms is becoming chaotic due to the fact that modern Russian linguistics often neglects the basic achievements of Soviet linguistics in the field of distinguishing language varieties. The article considers two approaches to the definition of vernacular, common slang and slang: from the point of view of their being fully functional language varieties and from the point of view of their functional facilities. As a result of the analysis of the data about language varieties, a conclusion is drawn about the common and different in these concepts. If these notions considered as similar to major language varieties such as standard language, standard colloquial speech and territorial dialects, then they cannot be called fully functional language varieties, since they have an extremely vague social base, they are characterized by a low degree of standardization, functional diversity and the intersection of their lexical content. Thus, it can be concluded that such language phenomena belong to the specific kind of language variations that are defined by their transitional nature. If functional facility of vernacular, common slang and slang is considered, it is worth noting that there are a lot of stylistically marked lexical units in slang. They are slightly less numerous in vernacular, and even less numerous in common slang. This peculiarity stems from the fact that common slang includes lexical units able to function as the fillers of the gaps in standard language, standard colloquial speech or territorial dialects. It should be noted that the lexical units of all three language phenomena are used in various communication situations even by the people who are well versed in the norms of the standard language. The fact that lexical units belong to vernacular, common slang or slang does not prevent well-educated speakers from using all stylistic functions of such units. Speakers who know the difference between standard and slang or vernacular words are able to vary these language tools to attract the attention of a certain category of people (for example, for advertising, communicating with youth, etc.).