Direct models use electricity to heat the water inside them. They use electric elements inside them to warm the water and are usually controlled by a timer and temperature control thermostat. Indirect models have a coil fitted within them which the water from the central heating flows around it warming the water and is controlled usually using the central heating timer and boiler controls. They come in a wide range of sizes and produce masses of hot water. Due to the nature of how they work there is no need for hot water pumps as the cylinders pressure is the same as the cold mains pressure entering the property.
Unvented hot water cylinders have to withstand considerable internal forces. For this reason most are made from stainless steel (as opposed to copper for a vented/gravity cylinder) utilising the tensile properties of the metal. This is why you should never try pressuring a cylinder unless designed for this purpose. To do so would introduce a very real risk of explosion.
NOTE: Unvented cylinders do represent a risk. They have a large volume of very hot water in a cylinder under pressure. If the contents of the cylinder were for example to get too hot (because of some system malfunction) the risk exists that the contents of the cylinder could boil, turn to steam and the internal pressure cause structural failure. We are talking about explosion! To combat this issue all unvented cylinders must be fitted with pressure relief equipment that will vent the cylinder in case of such a problem.