Difference between different classes of spirits.

Preta : Simply, it is a child that died in infancy and became a ghost. He can also be one who is deformed or misaligned or someone for whom the samskaras haven't been done for 10 days. If he dies he becomes a small Goblin.

Pishacha: This is the kind of guy about whom we listen from elders, don't be bad or you will become a wandering being. Usually comes from madmen, drunks, traitors or those who do violence.

Bhuta OTOH is a guy who died by accident or suicide or violence. Sometimes however Evil doers can also be a bhuta. Generally a bhuta will go and serve sm higher beings like devas and gandharvas but if they were notoriously evil they might be stuck in this realm. They generally reside in dry and deserted places and live on tall trees. The time after 12 am is considered easy for them to travel.