A diverter Valve or 3 way valve is a part that only appears in combi boilers, it allows the hot water to be sent to the central heating system (radiators, etc.) or to the hot water system (taps, etc.). This feature is what separates a normal boiler from a combination boiler due to the fact that a combi boiler wouldn’t require a cylinder to provide heating and hot water.
There are also external diverter valves, mid position valves and zone valves. These are commonly used on S plan systems and Y plan systems.