VU=complete overhaul of the ingame-model,probably also changes the desgin to some extent,comes with a new Voice over and new animations
TU=small texture update for the ingame model,most likely looks like just a better version of the current champ
MVU=Model-texture-update is basically a TU,but with slight model changes
TR=Texture rebalance,similar to a TU,but not as complex(usually just some colorations),lots of champions get these lately to make them fit more into the SR
Splash-art update=well just an updated splash art.Like you said there are currently a lot of new splash arts coming and it was said that most old splashes are gonna be updated(the ones that will be coming in the next patch are just the first to be finished),so rammus is most likely on their list too.