TEFL stands for Teaching English as a Foreign Language and it applies to teachers who work overseas, in a country where English is not the primary language. Those who teach English in the US or Canada, to immigrants, are ESL teachers or tutors of English as a Second Language. Other denominations for the same are Teachers of English to Speakers of Other Languages (TESOL) and Teaching English as a Second Language (TESL). Most names are interchangeable and use varies depending on the educational institution using it.
A TEFL certification is a course that allows people to instruct others in the use of the English language. The certification can take many forms, from online workshops to year-long courses. The most respected TEFL certifications are the ones given by The Boston Language Institute, the CELTA (UK-based) and the Certificate in TESOL (CertTESOL). All three can be obtained following a four- or five-week intensive coursework period. You don't need to be a native speaker in order to qualify for a TEFL certification, but your level of English should be native-like, both in writing and in spoken communication.
A certification course usually covers a series of topics, including language-teaching techniques, error correction, how to select appropriate material, lesson planning, teaching grammar and cross-cultural issues. Graduates from TEFL courses have a better idea of how to deal with students who speak little to no English, no matter their age. Because most graduates look for work overseas, the courses often include help learning interviewing techniques and contract negotiation.
Shorter certification courses, such as the ones that last a few weeks, usually cover the essentials mentioned above and cost anywhere from a few hundred dollars to a few thousand. Some more comprehensive courses of study, such as the California Teachers of English Learners Certificate, a graduate certificate program offered by the University of Phoenix, last longer. (This comprehensive program, which consists of four six-week classes and a one-week orientation, is designed to prepare teaching professionals to get the Cross-cultural Language and Academic Development, or CLAD, certificate.)
Because requirements vary from country to country, it is difficult to estimate what type of certification would be better to open international doors. Countries such as Japan ask for a CELTA plus a bachelor's degree in order to secure a teaching visa for you. Others, such as Russia, are happy with any type of TEFL certificate, in which case it may make sense to obtain a cheaper one. Asia, Latin America and Eastern Europe are main destinations for TEFL teachers.