Tourist Visa

If you are planning a short holiday in Thailand you may not require a Thailand visa if you are citizen of one of the approximately 40 nations that qualify for a Thai visa waiver or Thai visa exemption.  With evidence of onward/return travel, citizens of New Zealand, Australia, Canada, the USA, and most European nations may enter Thailand for up to 30 days without a Thai visa.  These Thailand visa waivers are issued upon arrival in Thailand.  Those entering Thailand by land may not receive a 30 day exemption however; the Thai visa waiver was reduced to 15 days for those entering by land after January 1, 2009.

If your citizenship does not qualify you for a Thailand visa waiver or if you are planning to stay in Thailand for longer than 15/30 days, you must apply for a Thai visa at an embassy or consulate outside of the Kingdom.  Thai tourist visas are valid for 30 or 60 days, though visitors who plan to travel from Thailand to a neighboring country and then back to Thailand can apply for several consecutive 30 or 60 day Thai visas.  At most, three Thai visas can be issued at one time, granting visitors either 30 or 60 days for each entry into Thailand (maximum 3×60 days, requiring visitors to leave within 60 days before beginning the next 60 day visa).

Source: http://www.tourismthailand.org/Travel-and-Transport/Travelers-Essentials/Passport-and-Visas