You can use them to trade with villagers, if you right click a villager a UI will come up and you can trade things.there are different types of villagers, such as librarians.
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You have to use a Iron Pickaxe to dig Emeralds ( Found in Extreme hills biome )
dimond
They are only in the 1.3.1 update and you have to look for them like you have to look for diamonds :P
If you are asking if you can just find weapons lying around in Minecraft, that is impossible. The two ways to get weapons in Minecraft are by, 1. Crafting them in a Crafting Bench, and, 2. By trading Emeralds to Villagers that are offering them.
you can either find them in extreme hills biome and extreme hills edge biome or you trade them with villagers. Answer from Andy.