It's very complicated and quite difficult to explain clearly and simply, but I'll try my best.
Use
Level to refer to
A1,
A2,
B1, etc. You can understand these as brackets of assessment criteria, as described by the CEFR. That means that when an assessing body has to rate your competence on a scale, it can place you within a particular bracket, which is usually for certification purposes.
Use
Stage to talk about where a learner is in his/her learning process. The words
Beginner,
Intermediate and
Advanced all describe stages of learning.
I know that sounds like
Level and
Stage are the same thing but they aren't quite. A good way to make the distinction is to understand
Level as an evaluation of somebody's
competence and
Stage as describing
where somebody is in a process. Another very simple way to think of the difference is to see
Level as useful for
assessors and
Stage as useful for
teachers.
Naturally, Level and Stage are matched closely (but not perfectly). Look at the following from the British Council website, which shows how they look at things. I've marked language related to
Level in blue and language related to
Stage in red.
The "Global scales" have three levels, and each level is divided into two:
A: Basic User
- A1 Breakthrough or beginner
- A2 Waystage or elementary
B: Independent User
- B1 Threshold or intermediate
- B2 Vantage or upper intermediate
C: Proficient User
- C1 Effective Operational Proficiency or advanced
- C2 Mastery or proficiency
Now, to add to the complication, teachers and course providers have their own ways of thinking about this, and these ways don't always match. In the past half decade or so, the idea that there is a useful distinction between a
Beginner and an
Absolute Beginner has begun to develop among many, in the UK at least. To accommodate this, we have introduced the descriptor
A0, which rather obviously can be placed directly before
A1. For those of us who do use this distinction, the word
Starter would describe a student at
A0 Level, not
A1.