Trespass to chattels is the unauthorized intermeddling with the use of a chattel of another. However, the line between possession and ownership may be difficult to draw in many instances. Hence, both claims may be available when personal property is involved. The main difference is that trespass to chattels requires a showing of actual and substantial damages, while conversion does not. In situations where the chattel has not been substantially damaged, a cause of action for trespass to chattels will not be available, while one for conversion would be.
Someone takes another person's car without permission and returns it.
The cause of action is for conversion. If the taker also destroys the car, trespass to chattels would also be available. If the person from whom the car is taken is merely holding the car for the true owner and the car is not destroyed, then the person from whom the car is taken does not have a cause of action for conversion, but the true owner would. If the car is destroyed, the person from whom the car is taken would have a cause of action for trespass while the true owner would have an action for conversion or trespass. Someone keys someone else's car slightly, causing an insignificant amount of damage.