You guys aren’t kidding right?
Well in a nutshell, lightcuts means the following: Imagine you want an expensive lighting rig in your scene, say, 7 area lamps. It would take a long time to render. But the light of those area lights could be approximated too with spot lights and regular lamps. Just thing about the spotlight dupliverted to a dome trick. OK, it could be a pain to manually substitute those expensive lights by ‘cheap lamps’. Well, light cuts does that automatically. And the best thing, no matter how complex and expensive you make your light rig, the rendertime stays about the same. Chances are you won’t notice a speedup if you use only a couple of expensive lamps. It is noticeable for complex (HDRI, IBL, Area Lights etc) setups.
GI: Notice how cheesy and sterile your image looks when you render it? That is because the CG lamps and CG objects are not physically acurate. They shine light on an object and the object bounces light back to the camera. But in reality, objects bounces light to eachother too, brightening eachother a little bit more. That is GI. That makes a CG image look ‘photo realistic’.