A war crime was an criminal offense that could be punished on a planetary or galactic scale. Genocide is an example of a war crime. Those convicted of a war crime were called war criminals.
Aamin Marritza was not on any Bajoran list of Cardassians wanted for war crimes, according to Odo, who claimed to have seen all of them. Later, posing as Gul Darhe'el, Marritza asked Major Kira Nerys "How could there be war crimes, when there wasn't any war?" during the Occupation of Bajor, claiming they had not fought, only surrendered. (DS9: "Duet")
In 2374, Dukat, former leader of the Cardassian Union, was to stand trial for war crimes before a Federation Special Jury at Starbase 621. However, he escaped custody and conviction when the starship transporting him, the USS Honshu was intercepted and destroyed by Cardassian warships. (DS9: "Waltz")
A year later, Dax and Worf, captured by the Breen and handed over to the Dominion, were informed that they would be tried, convicted and executed for unnamed war crimes. This sentence, however, would not be carried out – as the pair were being led to their execution, Legate Damar, having realized once and for all that the Dominion had little use for his people other than cannon fodder, committed his first act of rebellion, boldly gunning down their Jem'Hadar guards and telling the prisoners how to escape Dominion territory. (DS9: "Strange Bedfellows")
As a part of the Treaty of Bajor, the Female Changeling agreed to stand trial for war crimes committed during the Dominion War. (DS9: "What You Leave Behind")