The following designations are the most widely used to describe a professional certification of some sort in the field of English language teaching. There is no significant difference among them: it is often a matter of personal choice on the part of the institution offering the certification.
TEFL: Teaching English as a Foreign Language
TESL: Teaching English as a Second Language
TESOL: Teaching English to Speakers of Other Languages
What matters is the number of hours and the accreditation. In general, a 100-hour TESOL/TEFL/TESL certificate is more than enough to obtain an entry-level job worldwide. The most reputable ESL schools in the world ask for an advanced certificate such as TESL Canada, Trinity CertTESOL, CELTA, or equivalent. These programs consist of at least 120 hours of advanced study and a 20-hour Practicum module. TESL Canada recognizes some online course providers and CELTA recently launched an online course.
Coventry House International offers 5-week in-class Trinity CertTESOL and TESL Canada Professional Standard 1 courses. Ontesol’s 250-hour Diploma with Practicum is recognized by TESL Canada Professional Standard 2.