[Tulsi, Tulasi (india); Surasa (Sanskrit); Krapao (Thai); Hot Basil, Humong Basil (California Markets); Ocimum sanctum alt O. tenuiflorum (Mint family)]
In India Holy basils are highly revered, used for religious purposes and in Ayurvedic medicine. In Thailand they are used as a culinary herb, but unlike other basils, are always cooked, not used raw or just warmed. The terms Holy Basil and Tulsi are quite confusing as there are a number of rather different varieties, green (white), purple and in between. Unlike other basils the leaf margins can be strongly serrated, the leaves slightly fuzzy and the stems definitely fuzzy. They have very poor keeping qualities. The slightly rumpled photo specimens were purchased from a large Asian market in Los Angeles.
The three "recognized" varieties are Krishna (mostly to all purple), Vana, Sri or Lakshmi (green) and Rama (green with purple stems and veins). Rama, an annual, is the one most commonly found from herb vendors in North America. Vana (green) which grows wild in Southeast Asia is the one used in Thai cuisine.
The photo to the left is of Rama basil, purchased from an herb vendor at a farmer's market in Los Angeles. This variety is used mainly as a tea for therapeutic purposes. I see no reason it couldn't be used in cooking and have done so, but it isn't the kind found in Southeast Asia, and they don't use this sacred herb for for culinary purposes in India.
More on Basils.
Buying: Holy Basil, sometimes called "Hot Basil" or "Humong Basil" can sometimes be found in markets serving an active Southeast Asian community. It is highly perishable, so it is often already wilted in the market. It is best if you can get it at a farmers market, from a grower who specializes in Asian herbs. Of course, you can grow it yourself.
Storing: If very fresh and loosely bagged in plastic, this basil will keep up to 2 days in the fridge. If you buy it at a market you'll be lucky if it lasts long enough to get it home. If the leaves are only a little wilted you can refresh it somewhat by cutting the stem ends off and immersing the whole bunch completely in cold water for an hour, then dry in your salad spinner.
Substitutes: A number of substitutes have been proposed, including a mix of Thai Purple Basil and Mint or Perilla.
Cooking: Unlike other basils which are used mostly raw or just warmed as a garnish, Holy Basil is always used cooked, generally added early in the cooking process. In Thailand it is often an ingredient in stir fries, and also used in Vietnam, Laos and Cambodia.