There are many different types of heating systems in use. They differ slightly in design and how they are used.
Open Vented Systems
These systems have a vent pipe which is open to the atmosphere, terminating in a tank (usually in the loft).
On this type of system the central heating is pumped but the hot water works off the natural convection currents caused by hot water rising and displacing the cooler water.
This system operates fully under the influence of a pump. It does not rely on convection to circulate the hot water to the cylinder ( the hot water storage vessel) and can therefore have its boiler above or below the cylinder.
With these systems the hot water at the taps is the same pressure as the cold.
Combi boilers are so called because it incorporates the hot water storage vessel within the boiler. Along with the pump, diverter valve and some times the programmer.
Combi boilers are only found on sealed systems, but you can also have regular boilers as long as the manufacturer states so. If you convert to a sealed system it must have an expansion vessel fitted.