Curry leaves are the leaves of the curry tree, also known as Murraya koenigii. These leaves are used extensively in Southeast Asian cooking, adding a very distinct flavor and aroma to an assortment of dishes. Outside of Asia, the leaves can sometimes be difficult to obtain, although some markets carry them in dried form.
From the name, one might imagine that curry leaves smell and taste like curry powder. In fact, they are not related to curry powder at all, although both come from the same root, kari, which is Tamil for a stew of vegetables cooked in a rich sauce. While the leaves can certainly be used in curries and even with curry powder, they can also be used on their own in a variety of soups, stews, chutneys, and so forth.
At first glance, one could be forgiven for confusing curry and bay leaves. Both are elongated, pointy, and slightly shiny. However, their aromas and flavors are distinctly different. When sold fresh, curry leaves are often sold on the branch, allowing consumers to strip them off as needed. They can also be found frozen in some markets, and dried, in which case more may be needed to create the desired flavor.
Traditionally, curry leaves are toasted in oil in the cooking pan before any other ingredients are added. As they toast, the leaves start to release volatile aromas and flavors that will infuse the dish cooked in the pan. The leaves are left in the pan throughout the cooking process, and they can be eaten or set aside by diners. If you have ever eaten a Southeast Asian dish with small elongated leaves floating in it, you've experienced the curry leaf.
In some parts of Southeast Asia, curry leaves are chewed, because they are believed to be beneficial to digestion, and especially good for preventing diarrhea. You can also see them in some traditional herbal preparations, especially for the skin, as they are supposed to promote clear, healthy skin.
The flavor is extremely hard to replicate. Some cooks find success with basil or keffir lime leaves, one or both of which may be more readily available than curry leaves. For those who have never tasted the distinctive flavor of the curry leaf, replicating the flavor is obviously quite challenging, and ordering the leaves through a mail order company is strongly recommended so that cooks can get a feel for the taste.