Whitewater kayaking generally refers to sit-in plastic kayaks, where the paddler is sealed into the kayak by means of a neoprene spray-deck. Rolling becomes an essential skill when taking on higher grade rivers, as the consequences of swimming puts the paddler at risk. Beginner kayakers can take on lower grade rivers to gain confidence and work on their role, while they still have the option of a safe swim.
There are also sit-on-top whitewater kayaks that can make progression simpler and allow a transition to sit-in kayaks at a later stage if desired. Whitewater is far more technical than flat water and a course is highly recommended. You will gain greater enjoyment from the sport, the faster you progress and the more confident you become as a skilled and safety-conscious paddler.