Tamarind is a hardwood tree known scientifically as Tamarindus indica. It's native to Africa but also grows in India, Pakistan and many other tropical regions. The tree produces bean-like pods filled with seeds surrounded by a fibrous pulp.
The pulp of the young fruit is green and sour. As it ripens, the juicy pulp becomes paste-like and more sweet-sour.
Interestingly, tamarind is sometimes referred to as the "date of India."