Emeralds can be used for trading with villagers, as well as powering the beacon and building a pyramid-socket for it out of block of emeralds. They cannot be used to make tools. To trade with villagers, right-click them, and a trade menu will appear. To trade emeralds with villagers, the player must place them into the trading menu's left slot in the amount requested above the slot. The player can then take the resulting item from the right slot. This applies conversely to obtaining emeralds in trade. Trading is a much easier and less time consuming way to obtain emeralds than mining is.
Note that the emerald ore block cannot be obtained without the Silk Touch enchantment. If an emerald ore block is obtained, it is much more efficient to place the block and mine it with a pickaxe with the Fortune enchantment.