Neutral is the normal return path for current coming out of the live terminal and through the load. Ground is a safety wire that (all being well) should not be carrying any current. It is connected to the chassis etc. If some goes wrong inside the box and a live wire touches the chassis, the ground connection should prevent the metal parts going to high voltage. In this pathological case, the ground wire will carry a (possibility very large) current, at least until the fuse blows or the circuit breaker trips.
On a 3 phase system the neutral point is normally at the neutral center point of a wye or star winding. Normally it is grounded or connected to ground but not necessarily mandatory because there are systems wherein the neutral is not grounded for "ungrounded systems". On some systems the neutral may be "grounded". The star point is grounded by a grounding conductor but sometimes it is not necessarily require to run a neutral conductor from that point to the loads .
It is not only the star point that is grounded. Sometimes the corner point of a low voltage delta winding is grounded if a certain function is desired to be accomplished.
In single phase systems some transformer secondary winding has center tap that is grounded. Sometimes a neutral conductor is ran from this center tap winding to feed the loads that need the neutral connection.
The subject of neutral and grounding is very broad and has various / different requirements that depends upon how the system is designed to function.
Grounding is not only used in AC power system but also in DC systems. Grounding in AC and DC systems are also involved in the design of cathodic protection systems, lightning protection systems, static grounding discharge systems in ordnance facilities, grounding in hazardous locations NEC 500, and so on.