A variety of a language is a form that differs from other forms of the language systematically and coherently. "Variety" is a wider concept than style of prose or style of language .
Examples of varieties are:
Varieties, such as dialects, idiolects and sociolects, can be distinguished not only by their vocabulary, but also by differences in grammar, phonology and prosody. For instance melodic accent is differently performed in different dialects of the Scandinavian languages. As another example, foreign words in different sociolects vary in their degree of adaptation to the basic phonology of the language.
In certain professional jargons it might be useful to make distinctions of grammar, that are not in regular use outside of the jargon. For instance English journalists or lawyers often use grammatical moods such as subjunctive mood or conditional mood, which are no longer used frequently by other speakers.
It is a matter of definition whether slang and argot are to be considered included in the concept of variety or of style.
Colloquialisms and idioms are usually understood as limited to variation of lexicon, and hence of style.
