Minecraft Forge

Minecraft Forge is a Minecraft application programming interface (API) which allows almost maximum compatibility between other Forge mods. It contains a clean room rewrite of RML (Risugami's Modloader) which allows compatibility with modloader mods. It contains hooks into the base Minecraft files that allows modders to make their mods more advanced without editing base classes.

Source : http://www.minecraftforge.net/wiki/Main_Page