A variety of a language is a variant that differs from another variety systematically and coherently. "Variety" is a wider concept than style of prose or style of language.
Examples of varieties are:
- dialects, i.e. varieties spoken by geographically defined speech communities
- sociolects, i.e. varieties spoken by socially defined speech communities
- standard language, standardized for education and public performance
- idiolects, i.e. a variety particular for a certain person
- technical terminology, or jargon, with vocabulary or grammatical distinctions important in certain activities or professions
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 might journalists or lawyers be more prone to use grammatical moods such as subjunctive mood or conditional mood also in languages where these are considered foreign or archaic by other speakers.
It's a matter of definition whether slang and argot are to be considered included in the concept variety or in that of style. Colloquialisms and idiom are usually understood as limited to variation of lexicon, and hence of style.