Combi (combination) boilers supply your central heating and hot water. They use the mains to heat water instantly, meaning they do not need a storage cylinder. A combi boiler is a single, compact unit.
A back boiler is a compact unit fitted behind or beside a fireplace. They require a cold water storage tank and hot water cylinder elsewhere in the home. Back boilers are no longer being manufactured.
System boilers have a separate cylinder for storing the hot water, however they do not require a loft tank. Besides the cylinder storage unit, all of the heating and hot water components are built in.
Regular boilers are the traditional style and require a separate hot water storage cylinder and a cold water storage tank (usually stored in the loft). Older houses will likely use this boiler system.
Most homes will come fitted with a boiler as standard. However if you are moving into a new build and need to decide which unit will be best suited for you, select this option.