Citizens of a large number of nations (including EU, US, Canada, China, Japan, Korea, Australia, New Zealand, a number of South-East-Asian countries passport holders) enjoy visa-free entry to Thailand for up to 30 days.
Please check full detail at https://www.thaiembassy.com/thailand-visa/thai-visa-exemption-and-bilateral-agreement
Some others can obtain a visa-on-arrival at the airport for up to 15 days. Please check full detail of requirements, required documents, and conditions at https://www.thaiembassy.com/thailand-visa/visa-on-arrival
