To all intents and purposes, TEFL = TESL = TESOL, that is, teaching English to speakers of other languages.

The TEFL Certificate (aka TESL Certificate aka TESOL Certificate) is a qualification to show you can do this. The CELTA is essentially a high-level TEFL Certificate developed by Cambridge. In my opinion it's for teachers with a couple of years experience as it's very intense and crams a lot of information into a short course.