The heat source, circulation, emitters and flow/return loop that make up a typical domestic wet system.
A wet central heating system moves heat around a house using water as the carrier. Getting the basic loop clear in your head makes everything else, from controls to balancing, much easier.
The heat source, usually a boiler, raises the temperature of the water. A circulating pump then pushes that hot water around the pipework.
The hot water travels out along the flow pipe to the emitters, which are normally radiators. Each radiator gives up heat to the room, mostly by warming the air that passes over it (convection) with some radiant heat as well, so the water inside cools.
The cooled water returns to the boiler along the return pipe to be reheated, and the cycle repeats. So every radiator has a flow connection bringing hot water in and a return connection taking cooler water out.
Because the radiator nearest the pump would otherwise hog the flow, the system is balanced: the lockshield valve on each radiator is adjusted so that water is shared fairly and every room reaches temperature at a similar rate.
Modern systems are sealed and pressurised with an expansion vessel rather than fed from a loft tank, and they include controls so heat is only produced when it is needed. Use this as the mental model and confirm control and efficiency requirements against current Part L guidance.