Trespass to Chattels & Conversion
Prosser refers to trespass to chattels as the "little brother of conversion." While used more historically than it is today, trespass to chattels allows the prosecution of cases where the damage to an item or its holder is not sufficient to support a claim of conversion. As Prosser goes on to say, a trespass is, "Any direct and immediate intentional interference with a chattel in the possession of another." When trespass is found, a person can recover the value of the "lost use" of the item - and recover the item itself.
Conversion, on the other hand, allows a person to recover the full value of the item. Conversion can lie even where there are only nominal damages to the item. Conversion is a broader tort than trespass, as indicated by the quote above, and while it defies a solid definition, is essentially a determination by the court that the defendant's actions have rendered an item valueless to its rightful possessor - the item has been "converted."
Section 217 of the Restatement (2nd) of Torts states:
A trespass not amounting to a dispossession may be committed either by intentionally using or otherwise intermeddling with a chattel in the possession of another or by continuing to use or intermeddle therewith after a privilege to do so has been terminated.