What is the difference between a retirement visa and an extension of stay for retirement in Thailand?

It is the name, Retirement Visa also known as the Non Immigrant O visa, Non Immigrant O-A visa and Non Immigrant O-X visa which you buy at a Thai Consulate or Embassy. The Extension of Stay based on being over 50 years of age and sit out the rest of your life (NOT a visa and applied for at local Immigration Office) is also called retirement Visa. It is just easy to type retirement visa for all of those different Visa and Extension of Stay, but all have different requirements, rules and regulations.

Tod's post on 'retirement visas'

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And extension of stay (which is not a visa). It is something you get as the visa is about to expire that gives you persimission to stay in the country for another period of time. For the so called "retirement" extension, that period is usually 1year. You still need to meet all the requirements to get the extension. And there is no limit to how many you can get....as long as you keep meeting the requirements.

I don't know but if you use the "search this group" field (usually on the left), you'll find A LOT of info on this.