Foreigners from 42 countries can enter Thailand without a Visa for a temporary stay of 30 days. This countries include most Asian and European countries as well as the US! You will get your 30 day "Temporary Permit To Stay" in Thailand from the immigration officer on arrival.
People who would like to stay longer in Thailand should obtain a Tourist Visa from the Thai Embassy or Consulate in their home country. Tourist Visas are available for either 60 days with a single entry or 180 days with multiple entries. In the last case, you will have to exit and reenter Thailand after 60 days to get another 60 day stamp in your passport.
People who want to stay even longer or want to do business in Thailand, need a Non Immigrant Visa (cat. O or B) , good for 1 Year with multiple entry.
Each entry into Thailand with a Non Immigrant Visa gives you a permission of stay for 90 days. After that 90 days, you will also have to make a border or visa run. Exit and reenter Thailand for another stamp for 90 days.
Non Immigrant Visas category "O" can be extended for up to one year (so you don't have to make visa runs anymore) for people who are:
- over 50 years old and have at least 800,000,- Baht on a Thai bank account or a monthly income of at least 65,000,- Baht
- married to a Thai and have at least 400,000,- Baht on a Thai bank account or a monthly income of at least 40,000,- Baht
Please ask your local Immigration Office for more details.