A collaborator was an individual who was, almost always, willing to serve or sympathize with the people who conquered their lands. For this, they were often hated by their own.
During the Occupation of Bajor, several Bajorans served as Cardassian collaborators, including Secretary Kubus Oak and Basso Tromac. (DS9: "The Collaborator", "Wrongs Darker Than Death or Night") These collaborators either served the Cardassians directly, or provided the Cardassians with information on the Bajoran Resistance.
During the Dominion occupation of Deep Space 9, Kira Nerys initially worked alongside the Dominion in running the station in the belief that overt resistance would provide Dukat with the pretext to crack down on the station's Bajoran population. However, after Vedek Yassim killed herself on the Promenade in protest of the Dominion, Kira realized that, by making excuses for not fighting the Dominion, she had unknowingly become a collaborator. She and Odo thus began a resistance movement on the station. (DS9: "Rocks and Shoals")
In his war against the Dominion's occupation of Cardassia, Damar initially refused to attack any Cardassian guards, but ex-resistance fighter Kira countered that they were collaborators, and that the Dominion would place a Cardassian garrison at every critical facility once they realized that he wouldn't attack his own people. (DS9: "When It Rains...")