Something many new teachers may find confusing is exactly what kind of school to apply for.
In my few years in Thailand, I've both applied to and interviewed with a number of different schools (many of which were through Ajarn.com), so I thought it may help those new to the profession to clarify a few points, along with what general qualifications you need to even bother applying. I'm not discussing language schools here (read: mall-based English), only K-12 schools.
Please note that these are just MY experiences, and I'm sure others have had different experiences. Also, remember the golden rule that applies to all jobs: it's not necessarily what you know, it's who you know.
Although this isn't really fair judging by the quality of the students OR teachers, Thailand is all about image. In keeping with that (which also generally follows salaries), I'll rank them from what Thais usually consider least to most prestigious. Starting at the most common and most likely for first-timers in Thailand (aka, the bottom)
A Thai government school
Overview: this is the true "teaching in Thailand" experience, as it's the most "foreign" to most foreigners. In most schools, these are considered the "regular program" English classes. Responsibilities are generally relatively low, though, with the teacher often teaching 1-3 subjects/levels, just serving the position of "token foreigner."
Hiring: these schools will often hire through an agency due to high teacher turnover (due either to the school itself or the transient nature of teachers), although some hire directly with the school. Advertisements are common online (like this site), as few schools have their own [updated] websites.
Schools generally run May-September and October-March or so, so peak hiring seasons are typically April/May and October, although turnover often means you can get a job any time of year; Thai schools are notorious for doing things last-minute.
Qualifications: technically, these schools require only a degree in any field, although I've worked with several people who don't have degrees. Note, though, that most I know working without degrees are finding it increasingly harder to find teaching positions due to the government requiring degrees to get work permits, and in term, visas to live in Thailand.
A TEFL certificate is most often "highly desired" but not required; in my experience, it becomes something to differentiate who gets the job if two candidates are up and it's the only difference. That said, the TEFL can be highly helpful, especially for people with zero experience in teaching before. Those with degrees in education should look elsewhere.
Salaries/Responsibilities: in my experience, salaries here range from 25-35,000 baht (in Bangkok), and teachers teach (not counting actual hours in the school) generally 15-22 periods per week in non-air conditioned classrooms with class sizes ranging from 40-60 students.
Thai government schools with English Programs
Overview: government schools often have these dedicated English programs, often called an EP, MEP (Mini English Program), or some other derivation.
These programs operate as part of the overall public school, but consist of students whose parents pay for them to have dedicated English teachers (and air conditioning) for most subjects (usually alongside the same subjects in Thai). Generally, these programs are financial prerogatives of the school; the programs aren't necessarily that special for the students, but their budgets often pay for parts of the overall school budget.
Hiring: typically the same as a regular government school, although these programs are more likely to hire teachers directly with the school from advertisement websites (yet still often use agencies).
Qualifications: requirements are usually the same as basic schools (see above).
Salaries/Responsibilities: the students' English levels generally are slightly better than those in the "regular program," and teacher responsibilities for these generally 20-30 student classes are more than those teaching the larger classes. Salaries tend to be about the same or slightly higher than the regular programs, although teaching hours are generally less as teachers have to plan multiple different classes at different levels versus those teaching in regular programs.
Overview: considered a bit more "upscale" in terms of education (because you have to pay to study), private schools are a bit of a grey area, as they can widely vary in terms of who attends them and what subjects/areas they focus on.
In my experience, although the students often come from wealthier backgrounds than those going to basic government schools, teaching responsibilities, requirements, workloads, and salaries are roughly the same. The largest difference I've discerned (--and someone tell me if they've experienced otherwise) is that the administration in these schools is generally more hands-on (read: watching you) and the class sizes are more manageable.
Overview: these are an interesting lot; while they are usually also government schools, demonstration schools are attached to a university as a training ground for new teachers. Thais consider these to be upper-echelon public schools, and admission to the programs (especially for secondary school) is often competitive.
Hiring: these schools follow the Thai government school schedule for the most part, although I have come across one or two that follow their university's schedule (which typically follows a Western calendar). Friends of friends/acquaintances are generally picked through first, with advertisements on job websites usually following.
Qualifications: these schools generally will not hire a teacher without experience unless he/she has a teaching qualification (TEFL isn't considered such). You'll need a degree (undergraduate or graduate) in education and/or a good deal of experience to be considered for a position here, and teacher turnover isn't as high as at a normal Thai school.
Salaries/Responsibilities: starting salaries here are usually 50-65,000 baht/month, and often include benefits such as insurance and/or contract completion bonuses (extra month's salary, paid flight to visit home, etc.).
The responsibilities are more in line with what a Western school would want, with more modern teaching pedagogy used than normal Thai schools. That said, there will still be some large classes, depending on if you're in the English Program (or whatever they call it) or not.
To me, demonstration schools are trying to be more like Western/international schools, but are run by Thai-influenced Ph.D-holding administration.
Overview: slightly blurring into the private school realm are "bilingual" schools. As the name implies, these schools usually follow the same curriculum (often Singaporean or other Asian-based curriculums), and teach most subjects either in both languages (Thai and English/Japanese/Chinese/etc.), or primarily in the second language with most activities and reinforcements in Thai.
Bilingual schools are often the starting step for semi-qualified (i.e., have actual teaching licenses) teachers to eventually move up the ladder (see the next category).
Hiring: bilingual schools are a mixed bag. Sometimes, you can find an advertisement online (such as this website), but more often, the schools tend to advertise from within their own existing connections (such as friends/acquaintances of employed teachers) or from advertisements on their own websites.
Qualifications: although not always the case, bilingual schools generally want teachers with some sort of teaching qualification (more than a TEFL), setting these apart from government schools and many private schools. I do know of people working in bilingual schools with only a basic degree, but they have years of teaching experience.
Salaries/Responsibilities: salaries for these schools (in Bangkok, and in my experience) start around 40,000, and I've never come across a starting position above around 50,000. Subject-specific teachers (especially Math/Science) tend to earn higher salaries here, class sizes usually aren't more than 30-35, and the general atmosphere is a mix between a Thai school (read: disorganized) and a more Western-style school.
Overview: for career (long-term) teachers wanting to actually live in Thailand permanently, these are considered the Mecca of teaching. Curriculums and management are typically Western-based, and are basically adapted versions of the schools expats attended in their home countries.
Students almost always come from wealthy backgrounds as fees to these schools can be painfully high, and facilities are typically excellent (projectors in every room, pool on campus, etc.).
Within this category, though, there are what are considered upper-tier schools (read: expensive and respected), mid-tier, and lower-tier schools (blurring into the category of bilingual/private schools). The main difference in these tiers, to me, is the amount of money parents pay and the marketing the school does, just like any good business model.
Hiring: these are the hardest to get positions with, as [especially the upper-tier] schools usually prefer to hire teachers from their home countries and have them relocate to Thailand.
Calendars typically reflect the country whose curriculum is used, typically August-May, and hiring often starts well in advance; many of the top-respected schools begin hiring in November/December for the following August start date.
It is possible to get hired "locally" (already living in Thailand), but the schools seem to consider these teachers to be somehow jaded (in my experience), and local hires usually make considerably less and/or have less benefits than teachers hired abroad.
Qualifications: with few exceptions, teachers in the top international schools have valid teaching licenses in their home countries. This means teachers have undergraduate degrees in education and have completed the licensing requirements as well. A few years' experience is usually required too, although the mid and lower-tiered schools have been known to hire teachers straight out of university.
PGCE/QTS are typically a must for British-curriculum schools, and a valid state teaching license is a must for American-curriculum schools.
Salaries/Responsibilities: these are very similar to what a Western school would offer and expect; these are basically Western schools operating in Thailand. Small class sizes, extra-curricular involvement, regular parent/teacher conferences, and other school functions are the norm (in contrast with Thai-style schools).
Salaries are sometimes paid in the currency of the country of origin (although most often in Thai baht), and are on par with what teachers would make in their home countries. I've seen salaries ranging from 65,000 baht at lower-tiered schools all the way up to 120,000 baht per month (or more), alongside housing allowances, benefits, and bonuses. Obviously, this varies according to the school and the background of the applicant.
So, there you have it. Again, these are just my observations and experiences from the last few years; if you feel I've left something out, feel free to leave a comment!