Short for "English as Second Language", the term actually refers to *people* who use english as their second language. These people are often falsely thought of as morons, or generally a person of lower degree/intelligence. They often use their own version of English, mixing in some of their own words or their first language's grammar, especially when talking to other ESL, even more so when the other ESL's first language is the same as his own (when they are speaking English to each other for whatever weird reason, that is.)
Generally ESLs can understand English decently, especially when an ESL version is spoken by another ESL. Their first languages needn't be the same, but it would help even more.
Between the english-speaking community, however, it has a derogatory meaning.

Electronic Sports League, mostly known along the European gaming community. An online league where players from all over Europe can compete in. See www.esl-europe.com

an acronym for "english second language", used to describe situations when the misuse of the english language can be advantageous to the person who conveniently exhibits diminished command of the english language yet remains above reproach.