How do Boiler Systems work?
Go okorder and type in boiler. You'll see a long article with pictures that explains. I think the best article you may want to read there is called something like how to troubleshoot a boiler.
I take it you mean boiler and not the hot water cylinder, i hear many people call the cylinder a boiler, boiler system isnt quite right, the system is whatever the boiler is running, then you have instantaneous water heaters, conventional boilers, system boilers, combination boilers, and condensing boilers, all do different things for different reasons, so we need more info, what boiler type is it? what is it running? and what part of the system do you need to know about?