As a foreigner, you are required by Thai Immigration Law to initially enter the Kingdom on a Visa if you wish to have a long-term stay in Thailand

This can be a visa (tourist visa for 60 days or non-immigrant visa for 90 days) issued by a Thai Embassy or Consulate outside of Thailand.  You can also obtain a temporary stay with under the Visa Exemption Rule (VER). This allows citizens of 41 countries* permission to enter the Kingdom for a limited period of time (up to 30 days depending on the nationality) in accordance with Thai Immigration Law. This is commonly referred to as a 30-day stamp.

Under the new provisions of the Visa Exemption Rule (VER), visitors can now come regularly to Thailand without limiting their stay within a total of 90 days in a six-month period. Just keep in mind that under the VER, you are allowed 30 days maximum on each visit.

The restriction has already been abolished so tourists can now visit Thailand as often as they want, provided that they obtain 30 day Visa on Arrival at airports and 15 day Visa on Arrival if they are traveling via land borders. Thai Immigration Laws are revised on a regular basis for the convenience of tourists. However, immigration officials still recommend getting visas prior to arrival in Thailand, as they remind visitors that back-to-back short visas are not the proper way to extend the stay in the country on a long-term basis.

You may request for more time for your stay from Thai Immigration.  This is called an "extension of stay."   Typically, a short term extension of stay is granted up to 30 days for Tourist Visa or Non-Immigrant Visa.  Most people who come to Thailand want to extend stay.   They may apply for an extension of stay for one year, but it must be for one of the following purposes:   Business, Education, Marriage or Retirement.  If you hold a tourist visa, you must first convert to a non-immigrant status before the long term extension of stay.

However, not all foreigners who wish to enter Thailand are required to have their thai visas with them already. They can still enter the kingdom even without the possession of Thai Visas. The provisions regarding this matter is discussed in Visa Exemption Rule.