(covers information from several alternate timelines)
A temporal vortex is an artificially-generated rift in the spacetime continuum. Allowing for travel from one point in spacetime to another, temporal vortices have been known to have been used by species such as the Devidians and the Borg.
In 2368, the crew of the Enterprise-D discovered that the Devidians were using a temporal vortex to travel back in time to Earth, the year 1893, to harvest Human neural energy for sustenance. The vortex was formed by using an ophidian-like creature to create minute distortions in the spacetime continuum, and then using massive amounts of triolic waves to focus it to a specific time and place. While investigating on the planet Devidia II, Lieutenant Commander Data was inadvertantly pulled into the vortex, arriving in the 19th century. The rest of the Enterprise command crew, except for Lieutenant Worf, eventually followed, and were able to rescue Data. Returning through the vortex to the 24th century, they were able to destroy the Devidians' compound using a series of specially-modified photon torpedoes. (TNG: "Time's Arrow", "Time's Arrow, Part II")
In 2373, a Borg sphere generated a temporal vortex to the year 2063, through the controlled emission of chronometric particles. Pursuing the sphere, the USS Enterprise-E became caught in a resultant temporal wake, which had the effect of insulating the Enterprise from the changes in the timeline effected by the Borg. Scanning Earth, the Enterprise crew found that, in the new timeline, humanity had been assimilated, and realized that they had to follow the sphere through the rift in order to correct the damage done to history. Later, after having successfully restored the timeline, Commander Geordi La Forge was able to reconfigure the Enterprise's warp field to match the chronometric readings of the sphere, thereby recreating the vortex and allowing the ship to return to its own time. (Star Trek: First Contact)