These are the leaves from a variety of lime used often in Southeast Asian cooking. Kaffir limes (also called maktrut lime or mauritius papeda in Thai) are thick, shiny, and dark green, and have a spiced-citrus taste. If you can find them -- most often sold in Vietnamese or Asian markets -- they freeze really well! We have some friends who grow them locally and hooked us up with a large quantity of kaffir lime leaves. They live happily in my freezer until I made curry at home.