Minecraft Forge is one of the main API's in the Technic Packs. It supports a massive amount of mods and modpacks, including The Big Dig.
Minecraft Forge was created by LexManos. It is also compatible with all Modloader mods as it is a clean rewrite of RML (Risugami Modloader). The Forge connects with base minecraft files making it easier for modders to create advance mods but without editing the actual base files of the program.
You can download the Forge on it's on from: http://www.minecraftforge.net/wiki/Minecraft_Forge
But if you are/have downloaded the Big Dig pack then you have already got it, so don't worry about it!