Ender chests function the same as a single chest but instead they can be used as a "universal chest". If one ender chest is placed and is storing items then it can be accessible by spawning another ender chest. When the second chest is opened it has the exact same items as the first chest has and vice versa.
Ender chests can used between dimensions (Overworld, The Nether and The End). The chests produce a light level of 1 and purple particles emit from the chest like a nether portal. Though, regular chests can be made in to double chests ender chests do not share the same quality and thus, cannot be made into double.