There are a number of different options for entry into Thailand. The option you use will depend on how long and/or how often you want to travel to Thailand.
For the nationals of many countries it is possible to enter Thailand without applying for a visa before entering. For this type of entry all you need is a valid passport and proof that you have transportation organised to leave the country. It is possible to extend your stay in Thailand but only for up to 7 days and this can not be guaranteed. If you think you will be staying longer get a tourist visa before you leave home.
The list of countries covered by this scheme: Australia, Austria, Bahrain, Belgium, Brazil, Brunei, Cameroon, Canada, Denmark, Finland, France, Germany, Greece, Hong Kong, Indonesia, Ireland, Israel, Italy, Japan, Kuwait, Luxembourg, Macau, Malaysia, Netherlands, New Zealand, Norway, Peru, Philippines, Portugal, Qatar, Singapore, South Africa, South Korea, Spain, Sweden, Switzerland, Turkey, United Kingdom of Great Britain and N. Ireland, United States of America, Vietnam.
You can apply for a tourist visa before you arrive in Thailand. These are issued for stays of up to 60 days. They can be extended for up to an extra 30 days. Like with the non-visa entry extension you are not guaranteed to be given an extension.
If you would like to stay for longer you have a number of options. It is possible to get a multiple entry visitor visa. Your visa will have a maximum number of times that you can enter the country. Each stay is up to 60 days and at the end of each period you will need to leave Thailand before returning. There is no minimum length of time that you stay out of the country before you can re-enter.
Once your visa and/or number of entries have expired you can use the non-visa entry scheme to stay longer. During this time your ‘boarder runs’ will be 30 days apart. You can only stay in Thailand under this scheme for 90 days out of any 180 day period. Therefore you can only do 3 boarder runs before you will need to get another tourist visa.
There is another class of visas available for those wanting to spend an extended length of time in the country. These are the Non-Immigrant visas which are multiple entry visas. Each stay is up to 90 days before a boarder run is necessary. They are valid for 12 months. If your timing is correct, and you do a boarder run on the last day that the visa is valid, you can stay for 90 days after the expiration of the visa. The availability of these visas is highly variable with some consulates giving them easily and some being very difficult.
Many people over-stay their visas for various reasons. Most people seem to have no problem with this as long as you voluntarily leave the country ie you go to the airport or border crossing to leave the country. However, if you are caught in Thailand over-staying your visa you are in for an unpleasant experience. You will find yourself in jail until preparations are made for you to be deported at your own expense. Do not over-stay your visa!
If you do over-stay your visa and you leave on your own accord you may be required to pay a fine. There is no fine for an over-stay of one day. For two days the fine is 1000 Baht and every day after is 500 Baht.
Beware that both the day you arrive and the day you leave is one day each. Also if you go through immigration at 11pm but you flight is at 1am the following day it is the day that your flight leaves that is important.
For more information on Thailand and more specifically Pattaya you can try Pattaya Newbie.