Minecraft Forge is an application library for Minecraft. It is the foundation of the majority of current mods of Minecraft and developers can customise Mincecraft based on it. With Forge, Minecraft is limitless.
To do this, the staff of Forge have always updated versions to make mods compatiable with the update of Minecraft.
See also: Bookshelf API Library
How to install:
For Minecraft 1.8.9
For Minecraft 1.9
For Minecraft 1.9.4
Check out other Minecraft Mods