ESL (English as a Second Language) is big business and a lot of people are making money working in some part of this field. Here I'll introduce the four major players.
1. TEFL/TESOL/CELTA Providers
These companies provide teacher training to teach abroad via in-house or online programs. There are many programs as it is a largely unregulated field. All programs are not created equally and the quality of services that they provide can vary a great deal.
Recruiters act typically as a middle man. They find teachers for schools abroad and take care of the initial communication between the school and teacher.
In most cases they work on commission and they are paid when they find a teacher for the school. Usually the school involved will pay them, however, some recruiters may require a fee from you or a deduction from your first months pay. However, I wouldn't recommend this way if you use a recruiter as I think it is falling by the wayside.
In Korea recruiters make about 1,000,000 Won or more per teacher recruited. That's about $1000 or more per teacher.
Some of the largest companies (Hess, Pagoda, ECC) will have their own in-house recruiters. It's important to remember that they work on commission and that most recruiters haven't even been to the school and don't know much more other than the salary, work hours and other basic info.
3. ESL Teachers
Some teachers work abroad as volunteers and others can make a pretty good salary (up to $3000 or more a month with benefits). Most work for private or public schools and some may take up private tutoring where they can make more per hour. Some may also teach online.
4. ESL Schools
Schools range from public to private. Public schools often operate in a similar manner to what they do in your country. Private schools range from small mom and pops schools all they way up to giant companies/corporations with thousands of employees and hundreds of locations.
Hess in Taiwan, for example, claims to have 500 native speaking teachers island wide. Aeon in Japan, claims to have 3,000 employees at 300 branch schools in Japan with over 100,000 students.