Feed is the main nutritional source for your chickens. Hanging feeders will reduce the amount of food wasted from spillage.
Scratch is a mix of grains that is like junk food for chickens - no significant nutritional value, but it is interesting and they like it.
Grit is crushed gravel bits, which your chickens will need to digest anything other than their feed. They can get some grit from the bare earth if they are out in a pen with access to grass and dirt, but I believe most people put out a separate feeder of grit that the chickens will eat from "free choice" if they need it.