Tourist Visa
Before your trip, you can visit the Thai embassy in your home country and apply for a visa for up to 90 days. If you wish to stay longer than your permitted time, 60 and 90 day visas can be extended in any immigration office for a fee. You can only extend once and you get an additional 30 days.

Entry Without Visa
When a person enters Thailand without a valid visa, his/her passport will be stamped allowing access for a maximum of 30 days. Immigration officials may require proof of an onward ticket but such requests are rarely made.

Those with just a normal 30 day arrival stamp have to leave the country to receive an extra 30 days. This can be done as many times as you like. Literally walk out and walk straight back in.

Overstay
Please note that every day that you run over your visa will cost 500 Baht per day. Try to make sure you are near an exit/entry point as your visa is running out.