An unvented hot water cylinder is connected directly to the cold mains water feed eliminating the need for a header tank in your loft. Because this is connected directly to the mains feed it can deliver hot water at mains pressure which benefits you with high water pressure supply.
The cold water is then heated either from an electric immersion heater within the hot water cylinder (direct) or via the central heating boiler (indirect). This will determine if you need a direct unvented hot water cylinder or unvented indirect hot water cylinder. Twin coil hot water cylinders are used when you have two different methods of heating water.