The English language continues to spread rapidly as the world's 3rd most spoken language. Thus the need for English teachers worldwide. If this is a field you're thinking of entering then it's a good idea to get as much TESL information as possible first..
Everywhere, people are finding that they need to learn English to keep up in the postmodern world. Clients are coming in from everywhere thanks to the Internet and increased transportation networks and travel accessibility; and they might not be native English speakers, but English is often the one language held in common. Especially to avoid confusion, companies need their employees to learn proper email English, phone English, and even SMS (phone text message) English. Therefore the business branch of TESL is huge and in demand, often offering higher pay from the very beginning of your teaching days.
General English is what you will be teaching to the kindergarten through high school and college students, although ESL classes in the U.S. may have a higher number of adult students in this area of English due to the large amount of immigrants in the country needing to learn the basics.
Academic English is often taught among a high school and college crowd, where the students are hoping to pass entrance exams into English-speaking universities.
In some cases, you will be teaching students English for everyday use.
Sometimes maybe a more profession-oriented technical English (such as medical, legal, etc but only if you are qualified to teach such terminology), but in many cases, your students will be taking the course in order to prepare for a qualifying English exam that will help them enter into a university or get a better job or simply allow their current employer to assess their English abilities.
There are 3 different kinds of exams you will be helping students get ready for: general English, business English and academic English.