What does a hot water heater look like inside or what is the shematic of a heating system?

Not water heater look different depending on if they are gas or electric. Electric is a simple glass lined tank with a heating element or two screwed into the side. Its pretty much like a thermos bottle. What you see on the outisde is insulation and a sheetmetal jacket.

ballengerb1 gave you a great answer. Let me expand on it a bit.
I've put up a image of both a gas hot water heater and a electric one so you could see the difference. Cheers, Tom