There are many types of TEFL diploma course available. Some are conducted face to face, some in a blended learning situation and some completely online. However, there are only two which are widely recognized and independently monitored.
The TEFL diploma is for teachers who have already taught for a few years, perhaps even several years or longer. It is for teachers who wish to make tesol a career, not for those who only wish to teach and travel for a few years.
First - if you are in TEFL for the long haul, then it can help you develop your teaching. It will help you reflect on what you do in the classroom, it will focus your mind on the grammar, lexis, phonology and discourse of English. It will also help with teaching ideas, and allows the chance to focus on specialist areas of interest.
Second - it allows you to apply for better jobs. Better in terms of salary, and better in terms of moving into a different area of esl. For example, it may allow you to become involved in teacher training or materials writing. However, it's not a prerequisite for either.
It's most useful if you work in Europe or the Middle East. The British Council and some of the bigger, more established schools like their teachers to have a tesol diploma. Most esl jobs in the Middle East require the TEFL diploma - or an MA TESOL
If you work in South America or the rest of Asia - and plan to stay there - it probably won't make any difference to your tefl career, apart from in a handful of western owned schools. Whether it would be useful to take from a personal development perspective, you will have to answer yourself.
There are many types of TESOL diploma course available. However, from the point of view of career development, there are only two which are worth considering. Apart, perhaps, from certain university diploma courses. This is simply because in Europe and the Middle East, most employers only recognize these two types of diploma.
The DELTA - offered by the University of Cambridge ESOL Exams. DELTA stands for the diploma in English language teaching for adults.
The Dip TESOL - offered by Trinity College London. Dip TESOL stands for diploma in teaching English to speakers of other languages.
The Diploma in English Language Teaching for Adults is the bigger of the two. It's now available in modular form. The 3 modules focus on understanding language & language teaching methodology; planning and preparation of lessons - this module includes graded observations of your classes; and classroom management, syllabus design as well as a chosen specialism.
It's graded - in part externally - by a written exam, graded observations of your classes, and by and extended written assignment. It can be done full time over 8 weeks, part time over 9 months, or in a blended approach where the second 'practical' module is taught face to face, but the other 2 modules are taken online.
Cambridge English provide a lot of information on the DELTA, including the syllabus and example papers.
Trinity College London offer the Diploma in Teaching English to Speakers of Other Languages, or the Dip TESOL. It's widely recognized around the world. The 4 units focus on language, teaching and learning; a reflective portfolio of research; phonological theory; and classroom teaching, including assessed observations.
As with the DELTA, it can be taken full time on location over 8 weeks, often during summer vacation; or part time over 9 months. For more detailed information on the Dip TESOL, check out their website
Almost no employer will accept a - completely - online TEFL diploma. I don't mean a diploma with an online component and real teaching practice, as described above. Unlike online TEFL certificates , which sometimes have a benefit, the purely online TEFL diploma will only be of value in what it can teach you personally - but not in terms of career development.
If you are going to spend - a lot - of money on a TEFL diploma. Get one that is recognized around the world. Another option is taking an MA TESOL