Conversion is the act of taking possession or use of personal property or intellectual property to which you have no property rights or only property rights that do not allow such possession or use. It is an intentional tort. Although similar to the crime of theft (and indeed theft is a form of conversion), it covers a much broader set of circumstances that are related to situations such as repairs, warehousing, banking and pawning.
Conversion does not require dishonest intent or indeed any intention to deprive another of their property whatsoever. There is no valid defence of mistake of ownership (either from a mistake of fact or a mistake of law) to conversion the same way there is for theft.
In addition to the remedy of damages, conversion allows the remedy of detinue - an order for the return of the property to the rightful owner, even if it is in the possession of someone other than the tortfeasor who originally converted it. For example, if someone steals your painting and gives it to the museum in exchange for a charitable tax receipt, the original owner is entitled at law to the return of the property.
Conversion may be asserted for any act that is inconsistent with a person's ownership rights. For example, if someone takes your prize bull and turns it into steaks, that constitutes conversion of your property and you are entitled to the difference in value between the prize bull and the meat even though it constitutes the same physical property.
A similar tort with respect to real property is trespass.