Minecraft Forge is an API providing extra modding capabilities. These capabilities have been added to provide support for the different mods using the Forge. As of today, the forge contains. If you want to installed the Minecraft Forge API / compatibilty layers. You only need to do this once for all mods requiring it. Note that certain mod may include Minecraft Forge in their code, in which case you donít have to install the forge, but itís no harm to add it on top of the existing installation
Installation:

  1. Download ModLoader
  2. Download Minecraft Forge
  3. Open your minecraft.jar
  4. Drag the mod loader files over
  5. Delete META INF
  6. Drag over forge files
  7. Enjoy your Minecraft Forge

For Windows users, you just have to run install.cmd. If youíre on linux, you need to copy files and apply patch manually for now, A install.sh for linux users too is on the way.
This is a lot of changes, but, the big news about this update is that Mystcraft should now be able to release base clean versions, meaning it nolonger has to go in the jar!
Tutorial Video: How to install Minecraft Forge for Minecraft 1.4.6 | 9Minecraft | Minecraft Downloads

Changelogs:
v6.6.0

  • Update to Minecraft 1.4.7

v6.5.0
New:

  • New hooks to control map rendering per-world
  • New cloud renderer hooks
  • MASSIVE number of new terrain gen events to allow control over almost all aspects fo terrain gen
  • Decompile now provides proper packages for minecraft classes
  • FML and Forge classes are now signed, and mods can test if they are signed as well.

Bug Fix:

  • Kill install if astyle isnt found
  • Fix ForgeDirection.ROTATION_MATRIX
  • Creative GUI will now remember what tab page you were on
  • Make PlaySoundAtEntityEvent fire for players as well
  • Remove erronious double call to PlayerDestroyItemEvent
  • Fix up block drops for skulls and cocoa


For 1.4.7
For 1.4.6
For 1.4.5