Sociolects, as social varieties of language, should be classified and described on the basis of both linguistic and sociological criteria. In practice, however, most often only either the first one (e.g. according to Antoni Furdal and Danuta Buttler) or the second one is used (e.g. according to Aleksander Wilkoń and Tomasz Piekot). Both these criteria are less frequently combined (Stanisław Grabias). The article proposes those aspects of the sociological and linguistic functioning of language varieties that should be used by sociolinguistics especially in the characterisation of sociolects, but also in their typology. The sociological criteria include: the type of contacts in the group (including contacts made via the Internet), the degree of group formalisation, the durability of the group and the type of bond that binds the group together. Among the linguistic criteria the following were distinguished: nomination and expressiveness, the method of acquiring a sociolect and the degree of its codification and the rank of the linguistic norm. The article also clarifies the concept of the distinguishing features of the sociolectic vocabulary.