Thailand Visa and Customs Information
Citizens from 57 countries can enter Thailand without charge for 30 days without a visa. Countries include the U.S.A., U.K., Australia, South Africa, Ireland and New Zealand, as well as most European countries.
Passport must be valid for 6 months beyond your planned departure.
Tourist visas are good for 60 days and can be gotten through a Thai Embassy or consulate in your country. This can be done by mail. Once issued, the visa must be used (started) within 90 days of the date issued. The visa is invalid upon departure from Thailand even if the visitor hasn't stayed 60 days.
There are other types of visas but generally they don't apply to those visiting Thailand for a holiday.
Thailand's Ministry of Foreign affairs requirements for Thai Visas
Overstaying your visa is a serious matter and should not be taken lightly. On the one hand, you can clear up an overstay when you leave Thailand by paying your fine at the airport - 500 baht per day with a maximum of 20,000 baht. There is some recent discussion about jail time for serious overstays.
On the other hand, you are subject to arrest. Even if the police stop you for something else, if they find out you have overstayed your visa, they can arrest you. Being arrested is fairly common and it is extremely unpleasant. The Immigration Detention Centers are notorious for being not only highly uncomfortable, but extremely dangerous.
In addition to fines and detention, it is possible to be deported and black listed. That means you can not come back to Thailand.
Always make sure you are in Thailand legally.
Note: Recent announcements by the Thai Government state that Visa Rules are under review and some have been changed.
Visa based on Retirement in Thailand
List of Royal Thai Consulates