I would guess it's in the UK as I recognise the names. I am a bricklayer, not a plumber but can explain the basics and you will then probably know the systems by another name.
In the UK most systems are indirect with the mains going to the kitchen for drinking water and another branch going into the roof space to a cold water storage tank. This feeds the CW taps in the bathroom and a copper hot water cylinder which is vented back to the water tank. The water is heated by an electric element or indirectly by a copper tube heat exchanger from the central heating.
A thermal store is the reverse of this where the boiler heats the water directly in the cylinder and the cold water from the mains goes through the cylinder in a copper tube spiral heat exchanger and comes out hot at the top.
A megaflow is a pressurised cylinder which works off mains pressure and does away with the need for any tanks or plumbing in the roof space. The thermal store also does this as it usually has it's own header tant built in to it.
Tanks in the roof are potentially dangerous and have killed people when the stat on the heating element has failed. My wife keeps nagging me about changing our system but when I went up to change the ball cock last winter I noticed that it was over her side of the bed.