Definition: Tamarind is a sour, dark fruit that grows in a pod. While some cuisines use tamarind to make desserts and even candy, in Thai cooking it is used mostly in savory dishes. When combined with sugar, tamarind gives dishes a beautiful sweet-sour flavor. Classic pad Thai is made with tamarind, as are some Thai curries and seafood dishes. Indian curries also call for tamarind.
Pods of tamarind are available in many Asian stores. Note that the fruit has to be separated from the pod and that each pod contains a stone. The fruit is then boiled down to create a paste. It is more convenient to purchase already prepared bottled tamarind paste. In this form, it somewhat resembles molasses. Look for it in jars at your local Asian food store or Indian food store (I have had more luck finding it in Indian food stores). One jar will last for months and is very inexpensive. (See my photo)