There are three “registres” which are level languages. These levels depends of your public.
“registre élevé/soutenu”, “registre courant/normal” and “registre familier”.

See Wiki for more information

Registre Elevé / Soutenu

Use in literary circles or some speeches and publications, or with a interlocutor to whom it attaches importance.

Registre courant/ Normal

Simple vocabulary, including all single complex sentence. Grammar and syntax are met.

Registre Familier

The familiar level that is used among friends and family. Reserved for oral. Vocabulary often relaxed, sometimes vulgar or rude