Servers are a feature of Minecraft that allow players to connect and play together. All users are connected to the server by a Server Thread. If a server is forced to restart, all the villagers on the server will revert to their default, unaware minds. This almost happened in System Overload.
One of the first servers was Crafter's Server, which housed some of the very first villagers and monsters. This is where The Awakening occurred, and where Smithy of the Two Swords ✝ and Fencer lived. It is also the main location of the Great Zombie Invasion and Herobrine's first attack on Minecraft.