RO stands for Reverse Osmosis, and is a type of filtration done on tap water to make it pure. More or less, water is pushed through a really really fine membrane and the membrane only lets the pure water get through. People use RO (normally combined with a DI stage) to make pure water to add their salt mix to, and for freshwater top offs.
A UV sterilizer uses ultraviolet light to sterilize the water. It doesn't filter it, or take out any heavy metals - it just kills anything alive in it. UV sterilizers need regulated flow through them - if the flow is too fast, stuff isn't killed. People use UV to help kill off disease or any nasty things in their water.