There are so many types of chili pepper in Thailand. Different ones are used for different dishes.
The most famous one must be 'prig kee nu'. It's a very short, small green and red pepper. It is about one inch in length. It is the hottest of the pepper family. Prig kee nu is available in oriental grocery stores in the US. If you are visiting Hay Market, an open air market in Boston, it is available there, too. You can keep it frozen for years. I usually buy a large quantity and wash the pepper thoroughly. Let it air dry then put in a freezer bag and freeze until you are ready to use it. Just take what you need each time before using. It is a lot easier to slice when it is frozen.
'Prig chee fah', is not very hot and often used for its color and spice. The name literally means pointing toward sky chili. It is about3-5 inches in length. It comes in green and red.
'Prig yoorg', is pale green. It is not hot at all.