English as a Second Language (ESL) is the discipline of teaching English (usually to immigrants) in a country where English is the primary language (USA, Britain, Ireland, New Zealand, Australia, etc.)
English as a Foreign Language (EFL) is the teaching of English in countries where English is not the official language.
I'd like to go back to Mexico and teach EFL down there, because... well... I'm homesick for Mexico. It's a selfish reason but hey... you only live once. I'm a Spanish teacher here in USA (taking a little hiatus these last few weeks). I love Spanish, and speak perfect Spanish but I'm a native speaker of English and I don't want to take a job away from a national... so EFL it is!