A com-link was the link made between two communicators or to transfer data between non-linked computer systems allowed two or more devices to maintain a direct link remotely.
In 2153, during the exploration of a Delphic Expanse sphere that was used by the Osaarians as a warehouse, Captain Jonathan Archer determined that there were a lot of modules to search, so he instructed his landing party to "Fan out, keep open com-links." Later, Ensign Hoshi Sato used a com-link to hack into and download a Xindi database from the main computer of a hostile Osaarian merchant ship. In doing so, distance proved a critical factor, as the com-link was not stable over a distance of more than about a kilometer. Enterprise had to put itself at such close range for several minutes to avail itself of the com-link that it sustained heavy damage. (ENT: "Anomaly (ENT)")
In 2378, Captain Kathryn Janeway told Lieutenant Reginald Barclay that he earned a place in the history books for establishing the first trans-galactic com-link during Operation Watson. Barclay claimed he couldn't take all the credit since Harry Kim and Seven of Nine suggested bouncing a tachyon beam off of a quantum singularity to make it work.
Neelix hoped to transmit his holo-cookbook Cooking with Neelix, a Culinary Tour of the Delta Quadrant to the Doctor's publisher during his (Neelix's) com-link that afternoon. (VOY: "Author, Author")
Despite Janeway calling it a trans-galactic com-link, Operation Watson was not used to communicate with the USS Voyager beyond the Milky Way Galaxy as the ship was stranded in the Delta Quadrant and Barclay was on Earth in the Alpha Quadrant.