TESOL is an acronym standing for Teaching English to Speakers of Other Languages. Generally speaking it is the same as TESL or TEFL and it is used as an umbrella term primarily in the USA.
TESOL refers to the teaching of the English language to students whose mother tongue (MT) is not English. Typically these students live in an English speaking country (such as Canada or the USA) and need English for everyday use outside the home (where they probably use their MT for communication with other members of their family, etc).
The picture shows immigrants in 1980s New York. They came mainly from Europe and for them learning English was essential to settle in. Gradually they spoke it more and more beyond their mother tongue.
Whether you train to become TESOL, TEFL or TESL certified, as a teacher of English you will need to take into account your students’ needs, language background and local environment in order to prepare lessons which would be not only effective but also relevant to your students. A good teacher training course will give you the tools to achieve that, and more!
TEFL vs TESOL vs TESL – a look at the precise differences between TESOL and the other flavors of teaching English