English as a Second Language (ESL) implies the teaching and learning of English in an English-speaking region.
Example - A child moves to the United States from Mexico, and they begin learning English in an ESL class.
English as a Foreign Language (EFL) implies the teaching and learning of English in non-English speaking regions.
Example - A child in Russia is taking an EFL class in school.
Teaching English as a Foreign Language (TEFL) is the term used for teaching English in a country where it is not the primary language.
Example - A United Kingdom native going abroad and teaching English in China.
Teaching English as a Second Language (TESL) is the term used for teaching English to non-native speakers that are located in an English speaking country.
Example - An Australian teaching English to students that have moved from Japan to Australia.
Teaching English to Speakers of Other Languages (TESOL) incorporates both TEFL and TESL practices. A TESOL certification is known for having rigorous standards for teachers, a prescribed curriculum, and a minimum of six hours of supervised practice teaching in an actual student-classroom environment. A TESOL certificate is widely recognized by EFL teaching schools and programs as the mark of a well trained, highly qualified EFL teacher.
On the student side, the Test Of English as a Foreign Language (TOEFL) is a widely recognized test of proficiency in the English language. At most U.S. and U.K. colleges and universities, a verifiable passing mark on the TOEFL exam is a requirement to matriculate for all students from non-English-speaking countries.