A war crime is an criminal offense that can be punished on a planetary or galactic scale. Genocide is an example of a war crime. Those convicted of a war crime are called war criminals.
Aamin Marritza (posing as Gul Darhe'el) asked Major Kira Nerys "How could there be war crimes, when there wasn't any war?" during the Occupation of Bajor. (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 - though before the sentence could be carried out, Legate Damar, having finally realized 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")