It stands for English as a Second Language (ESL). It is the use or study of English by speakers with different native languages which is commonly known as second language learners or L2
English is a language which has great reach and influence; it is taught all over the world under many different circumstances. In English-speaking countries, the English language teaching has essentially evolved in two broad directions: instruction for people who intend to live there and for those who do not. These divisions have grown firmer as the instructors of these two “industries” have used different terminology, followed distinct training qualifications, formed separate professional associations, and so on.
Crucially, these data:text/mce-internal,two arms have very different funding structures, public in the former and private in the latter, and to some extent this influences the way schools are established and classes are held. Matters are further complicated by the fact that the United States and the United Kingdom, both major engines of the language, describe these categories in different terms: as many British observers have had attributed to them, the saying goes that “England and America are two countries divided by a common language.”
Cited from: http://en.wikipedia.org