What is the difference between neutral and earth?

Basically both Neutral and Earth/Ground are current carrying conductors in electrical system but in different situation.

 

Neutral Conductor:  Carries current in excess to the circuit but no potential/voltage(Zero Volts) based on normal conditions. This has a direct connection/node to a certain circuit.

 

Ground/Earth Conductor: Carries Current if a certain circuit has any line faults(open wire that touches either equipment housing and the like). This has no direct connection/node to a certain circuit.

 

the neutral wire is a fourth wire which is carries the unbalanced current in the 3phase.. the earth ground where the exist of current flow coming from the line it can be absorbed ang also ground wire is for safety purposes...

the neutral caries the current back from the load while the earth doesn't carry a current and it is connected to the body of the load for safety

The returning path of current is called "neutral".

When neutral is grounded it is called "earth".