The visa on arrival is a visa waiver. Certain nationalities can enter Thailand for a period of 90 days while others are allowed in for 30 days. There are also those who obtain 14 days on arrival while other will need to show money when arriving at an immigration check point. These do change so you can check with your local Thai Embassy in your country.
Visa on Arrival
The visa on arrival has changed over the years. Before you had been allowed to enter Thailand for 30 days. This was later changed as there was too much visa abuse where people had been living in Thailand for years on tourist visas. This was changed where they ended the visa runs. The process was where you would leave Thailand and return the same day getting 30 days again (Koreans got 90 days) or activating the next leg of your tourist visa.
Multiple entry tourist visas today are very difficult to obtain because of this. It was then changed that if you entered Thailand across a land border they would only give you 14 days on arrival. With this as well as cutting down on multiple tourist visas or how many times you could do this each year. It ended the visa run industry and everyone started obtaining a proper visa. You can see the TM.88 on this website.
30 Visa exemption on arrival**
Tourist Visa Exemption Scheme Andora, Australia, Austria, Bahrain, Belgium, Brazil, Brunei Darussalam, Canada, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hong Kong, Hungary, Iceland, Indonesia, Ireland, Israel, Italy, Japan, Korea, Kuwait, Latvia, Liechtenstein. Lithuania, Luxembourg, Malaysia, Maldives, Mauritius, Monaco, Netherlands, New Zealand, Norway, Oman, Peru, Philippines, Poland, Portugal, Qatar, San Marino, Singapore, Slovakia, Slovenia, Spain, South Africa, Sweden, Switzerland, Ukraine, Sweden, UAE, UK, USA, Vietnam
Bilateral Agreement arrangement Lao, Macau, Mongolia, Russia
(**Previous permitted period of stay was 30 days; the extension to 45 days will be in effect until 30 September 2021)
90 Visa exemption on arrival
Bilateral Agreement arrangement Argentina, Brazil, Chile, Korea, Peru
14 Visa exemption on arrival
Bilateral Agreement arrangement Cambodia, Myanmar
Thai Visa Options.
You are best advised to obtain a proper visa and not depend on the visa waiver. The Thai tourist visa as well as the retirement visa and marriage visa are all options. There is also the business visa if you are going to work in Thailand. See what the requirements are for this including those are are students in Thailand on an education visa.