Many people mistakenly talk about 'a TEFL' when they really mean to say a TEFL certificate. Then they go on to compare 'a TEFL' with a CELTA, when really a CELTA is one type of TEFL certificate, just as a poodle is one type of dog. See the article TEFL Certificate for a more detailed description of the different types of TEFL (or TESOL) certificate available. When talking about qualifications, TEFL and TESOL mean the same thing.
When I wrote the above explanation it was true–and still is for many people, but increasingly, TEFL and TESOL have become nouns to describe certificate courses. The only problem with this is that there's no established definition of which is which sort of course. So I stand by my original description, although in a few years I may have to rewrite this page completely. Incidentally, TEFL and TESOL are also sometimes used as verbs, as I did on the homepage. To tesol or not to tesol is still not standard usage :)
The acronyms TESL, TEFL and TESOL have slightly different meanings, but are usually used interchangeably, the same is true of ESL and EFL. On all other pages of this website I also use them interchangably. However, on this page I use them in the precise way.
Now, let's look at what the different acronyms actually stand for...
TEFL means teaching English as a foreign language.
TESL means teaching English as a second language.
The difference is that TEFL is used for teaching English in a country where the native language is not English. For example, teaching English in China, or teaching English in France. On the other hand, TESL is used to describe teaching English to people - usually immigrants - who are living in a country where English is the native language. For example teaching English in the United States, or teaching English in England.
However, to complicate matters, when students travel to an English speaking country for a short language course, this is often described as TEFL, not TESL. To simplify the situation a new acronym was invented - TESOL.
TESOL means teaching English to speakers of other languages.
But people still like to use the acronyms TESL and TEFL although TESOL is growing in popularity. TESOL sometimes has a more academic feel. University departments often prefer the acronym TESOL.
ESL and EFL simply mean English as a second language and English as a foreign language respectively.
If you look elsewhere on this website, remember that I'll be using the synonyms TESL, TEFL and TESOL interchangeably - as is common practice almost everywhere now.