CELTA stands for Certificate in Teaching English to Adults. TEFL stands for Teaching English as a Foreign Language. Essentially they are both qualifications to teach English as a foreign language, but there are a few differences.
TEFL refers to the general field of English language teaching, as well as the certification required to work in that field. CELTA is the name of a certification (accredited by Cambridge Assessment), but it can be thought of as a particular variety of TEFL.
TESOL stands for teaching English to speakers of other languages. It is a term which is used interchangeably with TEFL, but is more commonly used in the United States, while TEFL is more common in the rest of the world. A TESOL qualification is equivalent to a TEFL qualification.
There a few differences between the courses in terms of duration, accreditation and cost. What is important to understand is that CELTA is one specific qualification, while TEFL and TESOL are umbrella terms which can refer to many different TEFL and TESOL courses.
Read more: What’s the Difference between a CELTA and a TEFL?