The Swarm species was an unidentified territorial and hostile species which controlled a vast region of space in the Delta Quadrant as of 2373. According to Neelix, practically nothing was known about them. Most ships that crossed into their territory were never heard from again; some returned, but with the entire crew dead.
Although she knew that taking USS Voyager through Swarm territory would be dangerous, Captain Janeway decided to do so, since going around it would have added over fifteen extra months to their journey back to the Alpha Quadrant.
Their language was so complex and so unlike any language ever before encountered that translating it proved extremely difficult by using a standard Federation universal translator. The crew of Voyager never managed to translate more than the rough meaning of a few phrases. The Swarm species language consisted of a series of rapid clicks.
The Swarm species vessels were shuttle-sized, but, when working in large groups, could disable a bigger ship by swarming it, landing on its hull, draining power from it, and finally transporting a boarding party aboard. (VOY: "The Swarm")