Coconut milk or 'ga-ti' is traditionally made by mixing the grated meat of a ripe coconut with warm water and then squeezing out the juice. Nowadays, there is a machine that will grate a coconut into fine pieces that are like saw dust. Another machine presses the grated coconut and out comes the white, sweet aroma coconut milk. So you can go to a market in Thailand and buy fresh squeezed coconut milk.
For cooks outside of Thailand, coconut milk comes in a can or frozen in a plastic bag. Both canned and frozen are available at oriental groceries.
Canned coconut milk is more common and sometimes available at supermarket and costs $1 to $2 per can. Shake or stir the coconut milk prior to use. The unused coconut milk will keep in the refrigerator for a few days. Do not let coconut milk sit at room temperature for a long time; it tends to get spoiled easily. Sometimes with canned coconut milk, you can get a bad can that has oil floating on top of the coconut milk. This oil is a result of a can that got too hot during shipping and turned the coconut milk into oil.
Prior to my discovery of frozen coconut milk, I used canned. With frozen coconut milk, the taste is fresher and even the smell is frequently preserved (depending on the brand). The difference is particularly evident with desserts where the coconut milk's flavor is not competing with curry or other strong flavors. Coconut milk can be thawed and refrozen. However, when thawing coconut milk in a microwave or hot pan, do not let it boil.
What is 'lite' coconut milk? I have seen 'lite coconut milk' in a can recently and wondered how can coconut milk be light. The description on one can explains that it is the second press.
The first squeeze yields the creamiest of the coconut milk. It also contains the most flavor and scent. In traditional Thai cooking, the first squeeze of the coconut milk is used in curry dishes when first heating the curry. The first squeeze is also used in desserts. The latter squeezes are added to the curry to make the volume of broth (curry sauce). When I am making the broth with canned or frozen coconut milk, I dilute it with water if I feel that the sauce is too creamy or oily.
If you would like a lighter coconut milk, I recommend buying regular coconut milk, shaking the can and then diluting with water yourself. You'll save money and get a better flavor.