TESL refers to teaching English as a second language: programs in English-speaking countries for students who first language is other than English.
TEFL refers to teaching English as a foreign language: programs in countries where English is not the primary language and is not a lingua franca.
TESOL, which stands for teaching English to speakers of other languages, is a general name for the field of teaching that includes both TESL and TEFL. It's important not to confuse all occurrences of the acronym TESOL with TESOL International Association. For example, not every TESOL Certificate Program is offered by TESOL International Association. It may be offered by a university or even a private company.