The Plugin API is an upcoming feature that allows modders to easily add more content to the game. In contrast to mods, plugins will not require any alteration of the minecraft.jar itself. But they will also be slightly limited in their featureset. The Bukkit team will help make the official Plugin API.
The Plugin API has been planned in some capacity since at least July 5, 2010, shortly after the release of Alpha 1.0.1_01. The release was originally planned for 1.3, then for 1.4, and then it was accidentally stated by Curse that it would be implemented in 1.5, but has since been delayed.
You can get access to the new API here while it's being developed: https://github.com/Mojang/Minecraft-API
The following features are planned as part of the Plugin API:
- Plugins will be multiplayer compatible.
- It aims to make mods (to be known as plugins) easier to install and less prone to destroy your game.
- The Minecraft website will be revamped, helping to bring the most interesting community plugins and custom maps to the surface.
- It will be easier to add and animate models.
- Changing the way the game stores data regarding its many materials and items
At MineCon 2012 Mojang shared their vision for the future of the plugin API. The full presentation can be watched here: