Curry leaves (curry leaf ) is used in Indian cooking. The leaves look a bit like bay leaves along a long central stem and are used in a similar fashion. The pungent leaves are typically toasted in oil prior to the rest of the ingredients being added to the cooking pan. The leaves are best fresh but can be found frozen as well as dried.
The leaves have a short shelf life so if you can't use them right away you can pop them in the freezer and store them for a least a month.
This suggestion is from a Chowhound post on uses for fresh curry leaves:
my favourite way to add curry leaves to dishes is at the end, and with mustard seed. Heat a small amount of oil in a pan, add 1 tsp black mustard seeds (cover this, they pop), after they pop, add a handful of curry leaves...you can then add this flavored oil to anything as a finisher....it's a very nutty, herbaceous way to add umami to a veg. dish
If you don't have curry leaves, you can substitute one of the following:
You can purchase fresh or dried curry leaves in most any well-stocked Indian grocer as well as many Asian grocery stores. You can also purchase fresh and dried curry leaves at Amazon.com: Fresh Curry Leaves