A TEFL certification is the standard, international qualification that is required for teaching English as a foreign language worldwide. Teaching English as a foreign language refers to teaching in a country in which English is not the local or national language (as opposed to teaching English to non-native speakers in an English-speaking country). This is important because seeking TEFL certification implies that you wish to teach English in a foreign country.
Many organizations offer TEFL certification, including language schools and TEFL institutes. To meet international standards, certification should require a minimum of a four-week (or 120-hour) program of study. Both online and residential courses use the same core materials which include study of the principles and theories of TEFL and their application to the classroom environment. However, international standards also generally require a minimum of six hours of observed language teaching, which is not possible through an online course.
A residential program is an intensive course and provides the opportunity to practice teaching skills with real students. Programs run for four weeks and are offered in training centers worldwide. This type of program is ideal for someone who wants to obtain supervised classroom experience along with their TEFL certification. The other advantage to a residential program is that you may (depending on where you take the course) have the chance to experience a foreign culture, a plus for anyone considering a career in teaching abroad.
An online program can be studied wholly online, with or without a tutor. Through support materials, an online student can view actual lessons but will not have an opportunity to practice her own teaching skills in the classroom. The online course is self-paced and priced much lower than the residential program, which includes food and housing. Online programs can also be used to obtain advanced TEFL qualifications following the initial certification.
A compromise between the former two programs, the combined TEFL program involves online self-study of the basic TEFL principles, followed by a two-week course of classroom practice at a training center. The certification obtained at the end of the program is the same as that of the residential and online programs. As the cost of the program is not much lower than that of the residential program, the combined program is primarily for the student who doesn't have the time to attend the full four-week residential program.