Forge is some needlessly complicated, picky mod that's required for some mods to do...I don't know what. But most of the better mods use it.
I got it to work once, but one of the required mods is Optifine. My minecraft does not seem to like Optifine.
So it's like...You need to install about 3 mods, then forge AND THEN install what's needed for whatever mod you're using.
It's a pain :\
The newest version of Mo Creatures needs Forge. So I'm making the best out of the old version that I have before Minecraft updates.