Poes are ghosts in The Legend of Zelda series that are said to hate the world. As expected, you would normally find these enemies in the graveyards of the series as well as a few dungeons. They first appeared in The Legend of Zelda: A Link to the Past, they are enemies that you can find in the graveyard outside of Hyrule castle. The enemies will emerge from the grave occasionally after you push a grave stone.
Their next appearance was in The Legend of Zelda: Ocarina of Time. In this game, you were able to catch these enemies and sell them at the Ghost shop for rupees.
Most enemies in The Legend of Zelda: The Wind Waker underwent a major design change, and the same goes for the Poe species. Now, rather than being very dark they are multicolored and have masks on. Their trademark move is to possess Link and reverse the controls of Link (example, if you press left on the joystick Link would go right). To kill them, you must first shine light at their body, which will make them vulnerable. Now you can go up to them and slash them with your sword.
In The Legend of Zelda: Twilight Princess, there were 60 Poes that had to be defeated to get special items. The only way to kill a Poe in this game was to enter wolf form, attack it a couple of times, then rip out its soul.