Water boilers are essentially like big kettles. There is a heating element within them that heats your water to boiling. This water is then stored in an insulated container, ready to use whenever you need it.
When shopping for water heaters that plumb into your existing mains supply, you have to choose between a vented and unvented system. Because an unvented system doesn’t draw cold water into a separate holding tank, it takes up less room. They can also be quicker and more simple to install. They also tend to have better water pressure as they have the same pressure as your mains supply.
Vented water heaters provide instant hot water by drawing cold water from your supply and storing it in the holding chamber. Then, when you need it, it is moved from the holding chamber, over a heated element to deliver instant hot water. While this is happening, more cold water is drawn into the holding chamber for the process to repeat.