Teaching English as a second language (TESL) involves the teaching of English as a second language (ESL), that is English as a second language.
TESL generally refers to teaching English to students who need to integrate into an English speaking country. In this case students are studying ESL English as Second Language.
It can also refer to the practice of teaching English to students who live in countries such as India or Singapore, where English is an official second language. Under these circumstances it is sometimes called TEAL which is the acronym for Teaching English as an Additional Language.