This type of water/heating system includes a cold water tank, typically found in the loft or attic. It also features a hot water tank (often located in an airing cupboard). Gravity-fed systems can only accommodate low pressure shower systems. Therefore, showers requiring at least .05 bar minimum pressure will likely need a shower pump to work to their full effect.
A main water/heating system comes in the form of a combination boiler or an electric/gas water heater. Any shower system can operate effectively with a mains system.
A cylinder tank stores all hot water under pressure in an unvented system. These are most commonly found in airing cupboards or garages. Any and all showers are compatible with unvented systems.