TESL, or teaching English as a second language, refers to teaching non-English speaking individuals within a country that speaks English as a native language. For example, individuals who move to the United States from Russia would encounter teachers who practice TESL techniques because English is the common language in the United States.
TEFL, or teaching English as a foreign language, refers to teaching English within a country where English is not the predominant language. Instructors who teach English in a country like Japan, for instance, would practice TEFL techniques.
Individuals who are certified or have bachelor's degrees in TESL are typically found in countries that speak English predominantly such as the United States or United Kingdom. Individuals who have certification in TEFL travel to other countries where English is not the predominant language.
Though the education is predominantly the same for both certificates or degrees, those who seek a TEFL degree will typically have to know another language---primarily that of the country they will be teaching in---to use their degree to its fullest. Individuals who hold a TESL degree can be familiar with other languages but are not required to know a particular language fluently to be placed in a job. Individuals who are TEFL certified will also have to understand the different linguistics between the language they are teaching and the language the residents already know so that they can help their students learn English more efficiently.