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. (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.
The homeworld of the Mizarian species, Mizar II, was conquered six times in three centuries due to the species never resisting their conquerors. When Esoqq, Kova Tholl and Jean-Luc Picard were captured together in an alien cell, Esoqq suspected that Tholl might be a collaborator with their abductors. When he indirectly suggested this idea, Tholl asked outright if he was accusing him of collaborating with their abductors and he replied that collaboration was what his species did best. Later, when identifying each member of the group's roles in the "laboratory maze" that their captors had structured, Picard described Tholl as the collaborator who would defer to whoever has control. (TNG: "Allegiance")
In 2365, a Bajoran chemist named Vaatrik, who lived on Terok Nor during the Occupation, served as a direct link between Gul Dukat and at least eight Bajoran sympathizers (i.e. Ches'sarro Seeto). Vaatrik would be executed by Kira Nerys. (DS9: "Necessary Evil")
During the Dominion occupation of Deep Space 9, Kira 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...")