Indirect cylinders are generally configured to work in conjunction with wet central heating systems. Heat is imparted to the cylinder via an internal heat exchanger (boiler coil) through which hot water from the boiler flows (the heat source).
The contents of this type of cylinder is what appears at your taps.
These are referred to as both 'vented' and 'gravity' type cylinders because the cold water feed to them is supplied from a cold water cistern (often sited in the loft) under gravity pressure (low pressure). The cylinder is open vented and never under any more pressure than the weight of water from the header tank due to gravity.
Never connect a mains cold water feed to a gravity cylinder.