What are the key differences between a transit visa and a tourist visa for Thailand?

People from some countries get a 15 day visa on arrival for 2000baht when they land here and people from other countries need to get a visa from a thai consulate before they come here. If someone who qualifies for a 15 day VOA but wants to stay longer they can get a transit visa that gives them 30 days in country. It's common especially for countries that can't get in on a 30 day visa exempt entry or a 15 day 2000baht visa on arrival entry.

Transit Visa

This type of visa is issued to applicants who wish to enter the Kingdom for the following purposes :

To travel in transit through the Kingdom in order to proceed to the country of destination or to re-enter his/her own country (category "TS")

To participate in sports activities (sportsmen, sportswomen, etc.) (category "S") ***

*** Those who are scheduled to stay in the Kingdom longer than one month , Non-Immigrant Visa category "O" can be issued.

The person in charge or crew of a conveyance coming to a port, station or area in the Kingdom (category "C")

Note: Applicant is not permitted to engage in any occupation or employment.

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Tourist Visa

This type of visa is issued to applicants who wish to enter the Kingdom for tourism purposes.

Note: Applicant is not permitted to engage in any occupation or employment.

Holders of a Transit Visa may be granted entry and permitted to stay in the Kingdom at the discretion of the immigration officer for a period not exceeding 30 days.

Tourist visa are issued by consulates and for many passport holders, they are valid for 60 days. You can leave and come back (re -entry permit). And cost more than 1000baht.. I think you are think of a vise-exempt which you get when coming into thailand without visa -- that I know nothing about.