Curry Leaves are an essential part of coastal Indian cuisine, e.g. from Kerala.
Fresh curry leaves have a heavenly, citrus aroma and taste but without any fruitiness, as well as a mouth-watering acrid quality that stimulates the taste buds. Dried curry leaves have less obvious citrus notes, but these flavours are still definitely there and, if you cannot fresh easily, then these are a great addition.
Curry leaves are generally quite difficult to find fresh outside of the metropolises, but if you can get them they are a different beast to dried ones. Don't substitute with bay leaves, excluding them if not available.
Curry leaves are important for Southern Indian and Sri Lankan curries, and South African tamil cuisine. Use curry leaves in chutneys and pickles.