Curry leaf is not, unfortunately, the bouillon cube of the plant world - it does not magically turn an ordinary sauce into a curry-flavoured hit.
The thin, delicate leaves do, however, smell like Indian curries and, when used in a complex spice mixture, they add a strong fragrance and a distinct, slightly bitter flavour.
You can buy inexpensive bags of fresh curry leaves in specialist Indian produce shops, while pricier jars of dried specimens can be found in upscale supermarkets. Don't bother with the dried ones - usually, they've lost much of their aroma and flavour. And the fresh leaves freeze well.
In addition to being used in Indian kitchens - primarily in the south of the country - the leaves of the Murraya koenigii plant are used extensively in Sri Lanka and more sparingly in some Southeast Asian countries, including Vietnam, Cambodia and Malaysia. They are usually fried in oil, to deepen the colour and flavour.
Curry leaves are not reserved for curries. According to The Indian Kitchen, by Monisha Bharadwaj, ayurvedic medicine uses curry leaves to treat everything from digestive problems and hereditary diabetes to insect bites and premature greying of the hair.
Furthermore, they're used in chutneys, drinks, sautés and rice and vegetable dishes. For a delicious potato dish, heat a little oil in a skillet then add black mustard seeds and fry until they pop (this takes just a few seconds). Stir in some cumin seeds, add curry leaves and fry them until they darken. Remove the ingredients from the pan. Heat some more oil in the skillet, add some chopped onion and a little garlic and cook until soft. Mix in cooked, cubed potatoes, turmeric powder and chilli powder, then season to taste with salt. Stir in the mustard seeds, cumin seeds and curry leaves and cook until hot, then serve.