There are countless foreigners who have lived in Thailand for years and simply toddled off to a border point every 30 days, done a quick 'in and out' and received a fresh 30-day stamp. That all changed on October 1st 2006 when Thai immigration announced that you could only have a maximum of three 30-day stamps, and then you have to spend 90 days out of the country before you were allowed another one..........or you have to go to a Thai consulate abroad and get a 'proper' visa (tourist or non-immigrant) There are many theories as to why this rule came about but one popular notion is that it stops foreigners using the perpetual 30-day stamp as a cheap sort of permanent residency permit.

Tourist Visas: Can and must be issued by a Thai Embassy or Consulate and there are different numbers of entries (not always easy to get more than one though!). A single tourist Visa would automatically entitle you to 60 days on entry to Thailand, with the option to extend by 30 days (again at 1,900 Baht) at an Immigration office. A double entry tourist Visa would entitle you to the above, with a further 60 day entry once you've left and returned (exit and re-entry) to Thailand (see border runs further on) and the option to extend by a further 30 days. A triple entry would be the same but with THREE entries of 60 days etc. The most entries I'm aware of is four, but you'll find that most neighboring Asian countries will only issue singles or doubles at best. A common question is 'which embassy or consulate is the best to go to for a tourist visa? Penang in Malaysia? What about Kuala Lumpur? How about Vientiane in Laos? Or Cambodia even? The answer is 'who the hell knows?' You hear just as many success stories as you do refusals - from all consulates. It all really depends on which way the wind is blowing.

So let's recap on what you can do with a double-entry tourist visa if you're lucky enough to get one.
Let's say you enter Thailand on the 1st January (for sake of argument). You get 60 days on entry which will allow you to stay in Thailand until the 28/29th February. Just before the 60 days expire (or on the last day itself) you go to immigration office in Thailand and extend for a further month. Then you can extend for another two weeks (all extensions are currently 1,900 baht). When your final day of your final extension is almost upon you, you catch a bus, train or plane to a neighboring country's border point and then turn around and re-enter Thailand. Thus you now activate your second entry and your second 60-days. So in effect, a double entry tourist visa will get you 60 days plus 60 days plus an optional extension of one month on each entry and then a further extension of two weeks on each entry (all extensions can be done at an office within Thailand). So you would get about seven months out of your double entry tourist, but that includes one border hop and FOUR trips to the immigration office!
For the cost of tourist Visas in the UK go here: http://www.thaiconsul-uk.com
Remember - you cannot work legally in Thailand on a tourist visa!

Which brings us to Non-immigrant Visas! There are many many different types of these but I'll cover the main ones in the next section.

Again, the immigration rules overhaul of October 2006 meant that non-immigrant visas suddenly become a lot harder to get. Well, some people have had problems....but there are people who have problems doing anything.

A Non-immigrant O Visa is generally for people married to a Thai national or with Thai children or dependants. This works in basically the same way as a Non-imm B. Although if you can show funds in a Thai bank account (400,000 baht) or show sufficient proof of local or overseas income to prove that you can support your family, the immigration can indefinitely extend your Visa for up to 12 months. A work permit can now be obtained on a Non-imm O (it couldn't before 2006)

Generally (or it seems a lot of employers prefer) a Non-immigrant B Visa. Although it's possible to obtain a work permit with a Non-immigrant O Visa, it seems a lot of employers either aren't aware of this, or it's too much hassle for them. Generally you'll need at least a couple of months of your Visa left (which would initially be for 90 days) for the school to obtain all the needed paperwork...although if they can show they're going through the motions Immigration can and will grant an extension (generally of around a month) to give your employer time enough to finish everything off.

Important
Many people will disagree with me on this but I sincerely believe that no visa requirement rule is set in stone. There are many 'human' factors involved when you apply for a visa and you are face-to-face with an immigration officer.
Is the immigration officer in a good mood?
Does he / she like you as a person and are you polite?
Are you dressed smartly enough? (you don't need to be in a velvet smoking jacket but Thais like clean people)
Do you look like someone Thailand would want living in its country?
etc, etc - the list is endless. Most of it is just plain common sense.

Update from a teacher regarding the '7-day rule'
In my case, the employer wrote on whatever form it was that they presented to the Labour Department that my last date of employment was 12 June. They actually notified the Labour Department on 14 June and subsequently notified Immigration on 15 June. Immigration gave me until 18 June (ie, the clock started ticking the first second into 12 June) to leave the country. I was expecting a date of 21 June, so this was a bit of a surprise, but not a problem.