Gravity System (low pressure)
You will have a cold water storage tank,
usually in the roof space, and a hot water cylinder, often located in
an airing cupboard which is heated by a boiler, your fire (back boiler)
or an immersion heater.
Combination Boiler (high pressure)
This system heats your hot water as you use it. The boiler will often be in the kitchen and wall mounted, (But not always). You will not have a storage tank in the airing cupboard .
Unvented System (high pressure)
Here you will have a hot
water storage tank, usually in an airing cupboard, directly fed from the
mains water supply and heated by a boiler or immersion heater. The tank
will usually be quite large and may have a separate pressure vessel located
on top. There will be no cold storage tank in the roof space. This system
will usually supply higher volumes of hot water.