What's the difference between being accessory to a crime and conspiracy to commit a crime?

Conspiracy - agreement between two or more people to engage jointly in an unlawful or criminal act. Eg two people plan to rob a bank.

Accessory - aiding, contributing or assisting in an unlawful/criminal act. EG A decides's to rob B's house so C lends him some tools or his car to get away from the crime scene.

Conspiracy usually means that the perpetrators are involved in some way in the crime, usually in the planning/prep stage.

Accessory is usually where someone assists in the crime in some manner either before or after the event.

In both cases the conspirator or accessory will have knowledge that a criminal/unlawful act is about to be carried out/has been carried out.

Conspiracy means that person was involved from the beginning in the planning stage

Accessory means that person contributed in some way to the criminal event though they probably were not involved from the beginning

if a person loaned a vehicle or tools or other resources used in the crime

if they provided a place to hide or food etc etc they are accessories

Not much, Either way you are a criminal.