There are two types of cylinder a vented or unvented.

Vented Cylinder - A vented cylinder has a header tank which is usually situated in the loft space. The water feed relies on gravity from the header tank to pass the water through your pipes. A vented system is not really suitable for showers unless an additional pump is installed.

Unvented Cylinder - An unvented cylinder works off mains pressure water, providing you have good water pressure, this is the better option.

With an unvented system you do not need to pump water to showers, the cylinders are usually more efficient and have a longer life. The only downside to the unvented cylinder is the price.