A war crime is a criminal offense committed during the course of an armed conflict that violate the norms and procedures of a battle or laws of war of a particular state.
On Earth, the following offenses were among those named as war crimes by the Geneva Conventions: Willful killing, inflicting serious injuries on a person, torture, unlawful wanton destruction, forcing a prisoner of war to serve in the forces of a hostile power, depriving a prisoner of war a fair trial, using civilian shields, directing attacks against civilians, taking hostages, and summary executions. Genocide is sometimes considered to be a war crime, but is more often handled as a crime against humanity. The Dominion and its allies committed a number of such acts during the war, as did the Federation - specifically Section 31. (DS9 episode: "When It Rains..." et al.)
Following the surrender of the Dominion forces after the Battle of Cardassia, the Female Changeling was taken into Starfleet custody, and was imprisoned for committing war crimes. (DS9 episode: "What You Leave Behind")