The simple answer is that they are essentially the same.
 TESL = Teaching English as a Second Language. This term refers to teaching English in countries such as Canada,
the US, Australia, etc., where English is the common everyday language in the community.
 TEFL = Teaching English as a Foreign Language. This term to teaching English in countries such as Korea, Spain,
Brazil, etc., where English is not the common everyday language in the community.
TESOL = Teaching English to Speakers of Other Languages. This is a newer, umbrella term that encompasses both
English and non-English speaking countries.

>>>Wikipedia: http://en.wikipedia.org/wiki/ESL

There are many other acronyms used for teaching English. For example:
 ELT = English Language Teaching
 TEAL = Teaching English an Additional Language.
 EAP = English for Academic Purposes
 ESP = English for Specific Purpose

Other non-English Teaching acronyms:
 TOEFL = Test of English as a Foreign language (mostly used for University entrance)
 TOEIC = Test of English for International Communication (Mostly used for employment purposes)
 IELTS = International English Language Testing System (For university, immigration and other purposes)