Tamarind is a fruit used in dried form in a lot of Indian cooking. The usual way to buy it is in a compressed block consisting of fruit, skin and seeds, but it's generally used in recipes in the form of a paste, which is extracted from the dried fruit as follows.
Thanks to Madhur Jaffrey for this method.
Soak pieces of dried tamarind in hot water for three hours, then mash well with a fork and press through a sieve. Scrape out pulp remaining in the sieve, mix well with a little more hot water, and pass through the sieve again to glean any remaining flesh. Discard the remaining skin and stones from the sieve. Keep the resulting tamarind paste in a closed jar in the fridge for up to a week.
Note: If you need tamarind paste at short notice, an alternative to the three-hour soak is to simmer the tamarind in the water for 10 minutes, or microwave it in the hot water for 5 minutes, stirring often.