Also known as the "CEFR".
This is a document, published in initial form in 2001, that attempts to provide a reference point for what it means to have a certain "language level" - in any language. Unsurprisingly it isn't perfect, but it is effective and widely used by linguists, teachers and learners. according to the CEFR website, it's been translated into over 40 languages. It was developed under the aegis of the Council of Europe, which confusingly is not actually an EU organisation, it predates the EU and is larger. The Council of Europe is neither the European Council, nor the Council of the European Union. There are many things I would rather do than provide a description of functional relationships between the various institutions in Europe.
By providing a common standard for language learning, teaching and assessment, the CEFR seeks to enable:
effective learning and teaching objectives
redevelopment of language courses
teaching materials design
a reference for recognising language qualifications, facilitating educational and occupational mobility
These objectives paraphrased from the CEFR site
There are 6 level descriptors, in three categories. The brief summaries are adapted from the CEFR site:
Basic user - A1
Very limited and specific understanding and productive ability, needs significant help in a communicative setting.
Basic user - A2
Still restricted to the most commonly encountered language and contexts, able to produce and understand simple and direct information exchange. Needs some help from an interlocutor.
Independent user - B1
Has begun to move from universal general common topics to areas of personal or special interest. Can produce and understand more extended and less common language. Can begin to provide and understand deeper meaning beyond immediate requirements, e.g. reasons, opinions, explanations. Little to no help needed to successfully accomplish a restricted range of contextual goals.
Independent user - B2
Has developed fluency and accuracy far beyond a limited range of common contexts, is able to communicate main ideas in specialised areas (e.g. professional field). Can respond spontaneously to native speakers with no assistance and little noticeable effort. Can evaluate and give detail on a wide range of topics.
Proficient user - C1
Highly flexible user of the language. Able to engage with texts and contexts that are objectively demanding. Can structure a wide range of detailed discourse with a high level of contextual appropriacy in social or professional/highly specialised settings. Suitable candidate for high-level professional roles in the target language.
Proficient user - C2
Can effortlessly and effectively communicate in all contexts, inferring meaning whenever necessary. Consistently producing and receiving language with an extremely high level of fluency and accuracy, including subtle shades of meaning and levels of interpretation.
The CEFR also comprises highly detailed level descriptors across the skills, and "can do" statements that give a specific indication of what a language user should be capable of in order to be considered "at" a particular level. These are really useful for learners. There is a "Global Scale" designed to be used by non-specialists, a "Self assessment Grid" and a table that focuses on qualitative assessment of spoken language.
The CEFR is highly adaptable, and this writeup is not meant to be exhaustive. I have used the CEFR to design the syllabus documentation at a college I used to work at, I use it to assess and qualify my own level in languages I'm studying or practising, there are a lot of things I can do in French, that I can't do in Mandarin, for example. I also adapt it and circulate it among my students when appropriate for their own reflection and when we design tasks for classes together. It's very detailed and a great tool for language teachers and learners.
Hope this was a useful intro.