TEFL, ESL, and TESOL are all teacher training courses. Each of these qualifications is designed to prepare one for their English language teacher duties. The difference between these teach English programs is how they prepare instructors for different teaching environments. Depending on your international aspirations as an English teacher, one course may be a better fit than another. 

First we will look at the international TEFL training, TEFL stands for teaching English as a foreign language. TEFL certification aims to equip teachers with the skill set required to teach English to students, where English is not the native language. TEFL is for teachers who wish to work in countries that teach English as a second language. Therefore, during a training course for TEFL you will learn to handle a classroom of students learning English as a second language in a place where English is not commonly spoken. Due to the extensive amount of countries where this type of English language instruction is demanded and applicable, a TEFL qualification is the most widely recognized type of teaching certification in non-native English speaking countries around the globe. Additionally, earning TEFL certification from reputable institutions, such as respected colleges and universities, tend to be more highly regarded than TESOL certification courses. Education systems in non-English speaking nations all over the globe tend to have English teacher job vacancy for TEFL teachers. There is an abundance of jobs for TEFL teachers, which is another reason why it is a lot easier to find a job if you are TEFL certified than to try and teach English without certification. 

ESL stands for English as a second language. ESL certified teachers teach English to speakers of foreign languages living in a native-English speaking country. That is the main difference between an ESL certification and an ESL certification. Whereas, TEFL is best for teachers planning to relocate temporarily or permanently abroad, ESL teacher positions are suitable for those looking to expand their careers within their current home country, not for those that wish to be a foreign English teacher. Therefore, ESL jobs and ESL opportunities are more abundant in English speaking countries. 

The last certification we will look at is TESOL, and has not been around as long as the ESL English program or the certified TEFL courses.  A TESOL certification course combines both and provides the base for teachers to teach in both non-native English countries and within English speaking countries. Since TESOL is a newer certification, it can be less highly regarded than a TEFL, as overseas curricula tend to favour TEFL qualified candidates. Some schools and organizations may also not recognize a TESOL certification as a valid ESL teaching certificate.

In conclusion, the course you decide to take  depends on which type of environment you wish to teach in. TEFL is a certificate for overseas teachers of English or teaching abroad. ESL is for teaching in your home country or a native English speaking country. Lastly, TESOL combines both, but between internationally recognized TEFL or TESOL certification, TEFL is more recognized of the two.