TEFL is an acronym that means teaching English as a foreign language. It issued primarily to discuss teachers that will be working in a different country teaching and teaching English to nonnative English speakers.
The acronym ESL means English as a second language. This often refers to the students that TEFL teachers will be teaching.
Many programs offer TEFL training. TEFL training can be found online or in-class, there are even hybrid lesions that are in-class and lesions that are on location (this may be a class that trains you on the appropriate ways to teach that is located in a different country). Lessons can cost as little as $150 (for a 20 hour online certification course) or as much as $2,000 (for an in-class, on-location course). The training will cover different techniques for the most effective way of teaching English without knowing the native language.
No standard certification when you take a TEFL course exists. Some international schools looking for English teachers will look for CELTA certification (Certification in English Language Teaching for Adults), IDELT certification (International Diploma in English Language Teaching), Cambridge ELT, or Trinity certification. Both of these certifications are standardized, and they offer the school that you will be teaching at a sense of security with their certification. Several schools offer certification that are not CELTA or IDELT certification.
Several American universities offer a masters degree in TEFL. A master's degree is often needed for people who wish to teach in TEFL training courses or wish to work in countries that are more impacted with TEFL teachers, such as in Scandinavia and Switzerland.