TEFL stands for Teaching English as a Foreign Language. Simply put, this is usually used to talk about teaching English to people who live in a non-English speaking country and who want to learn English for business or to take an exam, etc.
It is pretty much equivalent to TESOL and TESL.
For more, see TEFL - Teaching English as a Foreign Language.
The ICAL TEFL Grammar Guide is a huge selection of articles which are all about English Grammar. They have been especially written for TEFL teachers and students and are presented in a user-friendly format.
The main areas are: