The Orb of Time was one of the nine Tears of the Prophets delivered to the Bajoran people by the wormhole aliens. Capable of transporting an object or person through time and space, it produced a characteristic surge of chronitons when doing so. The Orb was taken by the Cardassians during their occupation of Bajor, and was only returned in 2373.
In that year, the Cardassians, who were under siege by the Klingons at the time, contacted Starfleet with an offer to return the Orb. As the USS Defiant was transporting the Orb back to Bajor, Arne Darvin, a Klingon spy who had been surgically altered to appear Human, used it to transport the Defiant back in time to the year 2268, where he intended to change history by assassinating James Kirk. Captain Sisko and his crew, however, were able to foil Darvin's plot, while minimizing their participation in historical events. (DS9: "Trials and Tribble-ations")
Upon its return to Bajor, the Orb of Time was kept at the Temple of Iponu. In 2374, Major Kira, using her relationship with Captain Sisko, the Emissary of the Prophets, to gain access to the Orb, used it to travel back in time to 2346 and investigate Gul Dukat's claim that he had a relationship with Kira's mother, Meru. Upon learning that Dukat was telling the truth, and that her mother did indeed engage in a relationship with him, Kira attempted to change history by assassinating both Dukat and her mother. When she learned, however, that her mother was only staying with Dukat in order to secure her family's safety, she aborted the plan and saved Meru's and Dukat's life. Upon coming to terms with her mother's actions, she was returned to her proper place in time. (DS9: "Wrongs Darker Than Death or Night")
Use of the Orb of Time was controlled by the Vedek Assembly and the Department of Temporal Investigations. Using the orb to change history was forbidden and considered a violation of the Temporal Prime Directive which could have serious consequences. (DS9: "Trials and Tribble-ations", "Wrongs Darker Than Death or Night")