Bukkit is a free, open-source variant of the vanilla Minecraft server that allows programmers to extend it using plugins written in Java.
Bukkit is a Minecraft server variant that completely changes how running and modifying a Minecraft server is done, making managing and creating servers easier and providing more flexibility. It's focused on performance, ease-of-use and extreme customisability.
Bukkit provides you with the tools necessary to completely shape your server and makes it easy to maintain it through the use of a built-in plugin install, uninstall and updating system.
Plugin developers can write plugins to modify the game's behavior, adding new features into the game using the Bukkit API. They can then share their plugins with the community.
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As of Aug 21, 2014 at 8:42 AM, Bukkit has been discontinued and CraftBukkit (the actual server application) has been taken down due to DMCA requests.
Now, Spigot is a continuation of Bukkit and can be used in place of CraftBukkit.
Spigot
Spigot is a modification of the original CraftBukkit project, just made more efficient. It is a huge collection of various tweaks to help increase performance, and make it more flexible as server software.
It still implements the Bukkit API, allowing most plugins originally developed for CraftBukkit to be reverse-compatible with the Spigot platform.
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