The End is the shadowy 3rd Realm of Aetheria, and is made up of floating islands over the void made up of End Stone, much like The Aether. Enderman like Enderstar and Grayfellow are native to this Realm.
It is accessed by strongholds in The Overworld.
There is a central island with tall obsidian pillars and an enderdragon. This island is populated with Endermen.
Around the central island, there is nothing for at least 1,000 blocks.
There are end gateways used to access the outlying islands which stretch on for a long time. Here we find end cities which hold lucrative resources such as chorus fruit, potions, armor, shulker shells, and elytras.