A temporal vortex is a rift in the spacetime continuum that allows a vessel to travel from one point in time to another. Temporal vortices can be opened artificially, but are only open for short periods (long enough for two starships to enter) before collapsing. Their destination point in time can be precisely targeted and achieved.
In 2373, a Borg sphere opened a temporal vortex leading to 4 April, 2063 in orbit of the planet Earth. In pursuit of the sphere ship, the Federation starship Enterprise-E was caught in a resulting temporal wake, thus protected from the changes in the timeline wrought by the Borg incursion into the past. Following the Borg into the 21st century, the crew of the Enterprise was later able to recreate the Borg's temporal vortex and return to the 24th century. (Star Trek: First Contact)