What is the difference between a Vented and an Unvented Cylinder?

The main difference between a vented and unvented water storage cylinder is that an unvented hot water cylinder takes its water supply directly from the mains.

Whereas a vented cylinder is connected to a water tank by a vent pipe

You can get hot water out of an unvented cyinder at a much better pressure than a traditional vented cylinder as it is pressurised.