Curry leaves come from a range of tropical plants that are native to the Indian subcontinent. Curry leaves have a curry-like, slightly citrus flavour, but they are not a necessary part of any curry by any means; in fact, most curry mixes that we know of do not contain curry leaves. However, curry leaves are a welcome addition to any curry.
Many people find them a bit odd to cook with as you end up with chewy leaves in your curry (a bit like bay leaves), but they do impart flavour nonetheless.
Typical uses include: Meat, fish & vegetable dishes, soups, relishes and chutneys.