It differs from theft in that it does not include the element of intending to deprive the owner of permanent possession of that property.
An example might be tapping someone's secured wireless LAN or public utility line (which could also amount to theft of services).
Some have redefined such conduct as a specific type of theft, or another offence such as taking without owner's consent.
Note that the "unauthorized" use may begin after a period of authorized use, where, for example, a person rents a car then keeps it for an extra week without permission from the rental company.
Another common example occurs when a person fails to report finding lost goods (including animals), intending only to keep them until someone asks for their return.