TESOL stands for Teaching English to Speakers of Other Languages and is to almost all purposes the same as TEFL, Teaching English as a Foreign Language.
As they are basically the same we will refer to just one from here on: TESOL
Most TESOL teaching positions require a minimum qualification of a 120-hour certification. Throughout the industry there are three generally accepted routes to obtaining this qualification. These are through in-class courses, online courses or a combination of both, known as combined (or hybrid) courses.
Here the trainee will travel to a location where the course is offered. There are training centers in most countries throughout the world where English is being taught to speakers of other languages. The main advantage of an onsite course is the possibility of conducting some real life teaching practice. As the training usually takes place in language schools or language centers, part of the training often includes teaching practice and possibly being observed by your trainers. This experience is extremely useful, especially if you have never taught before.
Online courses have the advantage that you can take them from your own home, without the need to travel to a new location. This makes the course less expensive than an onsite option. You will generally cover all the same material as that of an onsite course. Some courses now offer virtual teaching practice additions to their online courses, so you can get a feel for online teaching, in the real world. A very large part of the TESOL teaching market is online, so this experience is very useful.
Sometimes also referred to as hybrid courses, combined courses offer the main aspects of the previous two modes of learning. You usually cover all the theory online and then travel to a worldwide location for some additional teaching practice. As the requirement to be in the onsite location is typically only eight to ten days, rather than 4-weeks, as with the onsite course the combined courses are typically cheaper than the 4-week onsite version, but more expensive than the purely online version.
There are three typical working scenarios you may find yourself in:
1) Online: You can work remotely from wherever you choose to be. There are now many companies that work in this way. Contracts and hiring requirements vary greatly, so do your research carefully before signing a contract.
2) Onsite: Typically you teach onsite to a range of class ages and levels. This would be at a fixed location, such as a school or language center. Usually this requires you to get a working visa for the country you want to work in.
3) Combination: Finally, you may be able to find work in both the previous scenarios simultaneously. Short-hour contracts are possible with two different teaching positions, something that is becoming a popular route.
Whichever option you choose, contracts are typically limited to one school year. If you find the position does not suit you, you can always move on once your contract expires.