There are two main internationally recognised qualifications: The Trinity TESOL and Cambridge CELTA. There are a number of other courses available, but you should make sure that the course is externally assessed and has a certain amount of monitored teaching time.
This certificate course is often considered to be the reference for TEFL qualifications and is widely respected internationally. The full name is the Cambridge ELT Certificate in English Language Teaching to Adults (CELTA). This replaces the previous Certificate in Teaching of English as a Foreign Language to Adults (CTEFLA). CELTA places great emphasis on teaching practice. With over 8,000 enrolments worldwide per year, CELTA is clearly the most popular TEFL course among teachers. It is externally validated by the University of Cambridge Local Examinations Syndicate (UCLES).
CELTA courses are usually 4-week intensive programmes although it is sometimes possible to take the course on a part-time basis. In all CELTA courses, observed teaching practice is an important and integral part of the programme.
Entry requirements for CELTA courses vary somewhat from centre to centre. Although the course is open to native and non-native speakers, all candidates must be interviewed and take a language awareness test. Some centres require candidates to be graduates while others accept candidates with a good general education and command of English.
There are more than 200 centres offering CELTA courses in over 40 countries around the world. The course cost varies from about £500 to £1,000 ($800 to $1,600) depending on centre.
About 4,000 candidates annually qualify for the Trinity College London Certificate in Teaching English to Speakers of Other Languages. The Trinity syllabus is strictly controlled with basic requirements common to all validated centres. All trainees are, for example, expected to experience learning a foreign language.
The course may be full-time, usually following a 4- to 6-week intensive programme with 130 tuition hours, or part-time over a number of months. In many Trinity centres, it is possible to take a short, professional course as an introduction, while some centres also offer a correspondence module in addition to the main programme.
Entry requirements vary between a good educational background and a university degree, with a good command of English obviously being a prime requirement. No distinction is made between native and non-native speakers.
Most Trinity Centres are in the UK although there are now a growing number overseas. The course cost varies from about £500 to £1,000 ($800 to $1,600) depending on centre.
There are various other TEFL-related courses, some of them preparatory, some specialized, some run by chain schools who wish to train teachers in their own particular methods. Short, introductory courses can be useful to help you decide whether TEFL is for you or in the event that you cannot afford a full certificate course. Specialised courses (eg business English or neuro-linguistic programming) are useful for experienced teachers who wish to develop their careers.
(The above information is courtesy of www.tefl.net)