The 6 levels of language proficiency are:
A1 - Beginner
A2 - Elementary
B1 - Intermediate
B2 - Upper Intermediate
C1 - Advanced
C2 - Mastery
I like the language used in the PDF document found at Europass' website in outlining what each level looks and sounds like. You can download it here or select the component you wish to elaborate upon and view the criteria for each language level directly on the website.
My posts on this website are often labeled according to this framework to assist teachers, parents, and language learners on finding resources appropriate to their language learning proficiency.