QUESTION: What is the difference between an under sink unit and an R.O. system?

ANSWER: An R.O. system is a Reverse Osmosis unit, which usually combines a sediment and carbon filter with a membrane separation unit that removes contaminants through crossflow filtration down to ionic levels, producing 99.9% purified water. Typically, it's installed under the sink. However, other units also are frequently installed undersink as well, from basic filtration to more complex systems that incorporate ultraviolet or ozone disinfection. Distillers are usually countertop units.
Hope that helps.