There are many types of basil available here in the U.S. There only 3 kinds that are used in Thai cooking, Thai basil, holy basil and lemon basil.
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Thai basil or 'ho-ra-pa' is different than the sweet basil or Italian basil in supermarket. It has smaller leaves with purple stems. It is available at oriental grocery stores and farmers markets. If you can get some seeds, it is very easy to grow. Choose a sunny spot, you will have you Thai basil all summer along. Pick often to encourage growth.
I often get hora-pa mixed with gra-pow, holy basil. My mom taught me how to remember which is which by noticing the hair on the leaves and branches. The one without the hair is hora- pa and the one with hair is 'gra-pow'.
Lemon basil or bai mang lug looks similar to Italian basil but has tiny hair on its stems. The leaves are smaller. It definitely smell "lemony". Seeds are soaked and eaten as dessert. When soaked, the seed forms a tiny cloud and looks like frog's egg. The dessert is served with coconut milk and syrup.
Holy basil is also known as tulsi in India. I have heard a story that Thai people moving to India are warned not to pluck the leaves from the plant without seeking permission as they are holy plants.
Holy basil is gapow in Thai. There are several varieties in Thailand. Some are grown wildly. If you trek in Thailand, check out for the reddish, purplish variety. It can be grown from seeds in the US in the summer.
You are probably familiar with 'gai pad gapow'. Frequently here in the US, when you order 'pad gapow' in a restaurant, you do not get the real gra pow, but instead sweet basil. They are not the same. However, if push comes to shove, sweet basil is better than nothing.
Gapow has a different smell and flavor than basil. Its branches and leaves are covered with tiny hair. It has a hot spicy smell. If you are making 'pad gra pow', don't sacrifice the taste, use all that you've got. It is worth it.