A Turn is the way of giving structure to a Duel.

Every turn is divided into the six following phases:
Draw Phase
Standby Phase
Main Phase 1
Battle Phase
Main Phase 2
End Phase

You have to go through these phases in the given order until you have finished with your End Phase. You can skip the Battle Phase, and if you do you can't conduct Main Phase 2.
You can always skip Main Phase 2.

A turn can only begin when a player conducts a phase out of the six phases, if not changed by a Card Effect you have to start with your Draw Phase. If through a Card Effect you have to skip your Draw Phase, your turn starts with the Standby Phase.
A turn can only end after an End Phase.