The Swarm species vessel was a vessel used by the Swarm species to protect their space from foreign vessels.
The USS Voyager encountered numerous Swarm vessels in 2373 while attempting to cross their space. Initially, a single vessel was detected leaving a freighter belonging to a Mislenite named Chardis. This vessel disengaged Voyager's stealth shield modifications and alerted other Swarm vessels to the presence of Voyager.
Rather than attacking directly with weaponry, the tactic of Swarm vessels against larger vessels was to literally 'swarm' a ship, simultaneously draining its energy and partially crushing it, disabling vital systems. Members of their species would attempt to transport on board the vessel under attack.
The ships were shuttle-sized, with a weakness being their connectivity with other Swarm vessels. If a feedback pulse was fired at Swarm vessels, other nearby vessels would also be damaged. (VOY: "The Swarm")