@Rick ******
visa exempt means you don't have any visa at all. You are simply stamped in for 30 days when you arrive, and you can extend this for 30 more days. It's based on an agreement between Thailand and specific countries.
Visa on arrival is when you PAY for a visa when you land in Thailand. These usually cannot be extended and are much more limited than visa exempt status. Sometimes they are only good for 15 days depending on the country you arrive from.