A writ of execution is a court order that gives effect to a possession order and directs law enforcement officials to begin transferring assets, money, or property as a result of the judgment.
The decision on possession states that the plaintiff has a right to the property; the writ of execution actually begins the process of transfer from the debtor to the plaintiff.
A writ of execution can be used in bankruptcy cases and in eviction cases (when the tenant does not move out and does not pay rent).