If you are easy then you get 25% added leadership, 25% more research points, and you use 25% less supplies. Very easy doubles these. If you are hard, you use more supplies. Easy AIs have less organization for their units (I think) but hard AIs get numerous bonuses like all of their pops militancy ratings have a -0.05 a month, they use less supplies, have a badboy reduction per month (-0.05), and get more research and leadership. I don't know the entire details (because I've screwed with mine so much) but you can see them in the static_modifiers section.