The exams Teaching English as a Foreign Language / TEFL and Teaching English to Speakers of Other Languages / TESOL have no official significance, but are used as general terms for teaching certificates. You should ensure, that the school offering a TEFL or TESOL preparation course cooperates with the UCLES (University of Cambridge Local Examinations Syndicate) or Trinity College London. These two are worldwide accepted for the TEFL and TESOL tests and exams. Accredited courses are the following:

Validated by UCLES:
Cambridge CELTA (Certificate in English Language Teaching to Adults)
Cambridge DELTA (Diploma in English Language Teaching to Adults)
COTE (Certificate for Overseas Teachers of English)
CELTYL (Certificate in English Language Teaching to Young Learners)
Validated by Trinity College of English:
Trinity Certificate TESOL
Trinity Diploma TESOL
Trinity Certificate TEYL (Certificate in Teaching English to Young Learners)

English teaching certifications can be earned through an intensive 4-week program, or a longer part-time program which is in turn internationally-recognized qualification accepted by many employers around the world. The Cert TESOL and the CELTA are two internationally-recognized programs and are accredited in the UK on the National Qualifications Framework. Both qualifications are externally assessed and accepted by the British Council in their accredited teaching organizations worldwide in 100+ countries.

Depends on course.