The Swarm was a fleet of linked units that numbered in the thousands (to be precise nine hundred twenty seven thousand six hundred and eight) when Commander Spock first discovered the Swarm with the scanners of the USS Enterprise, a number that was still increasing as he informed Captain Kirk about them.
These units traveled from star system to star system, using chronometric radiation to rapidly age stars to the point that they would expand or go nova, and then harvest the released stellar matter to refuel themselves and create more units through a process Commander Spock described to be similar to a Starfleet transporter, converting energy into matter.
Each unit resembled a crimson red "horseshoe crab" as described by Helmsman Hikaru Sulu. The units in general had an average length of twenty-five meters though some units were larger and some units were smaller.
The Swarm used a propulsion system that folds space as Lieutenant Commander Scott described their main star drive. When activated the drive generated a shockwave that at close range could tear even a shielded Constitution-class starship apart.
They were heavily armored as they were designed or had evolved to resist the destructive result of a star going nova at close range, this made the units also invulnerable to Starfleet issue phasers and photon torpedoes. The units did however have one weakness, their maneuvering engines which consisted of standard chemical rockets that were not armored and could be destroyed by heat seeking photon torpedoes.
When the units determined the Enterprise to be some type of threat, a number of them engaged the starship with ionic pulse weapons. They also bombarded the Enterprise with chronometric radiation, and as a result the ship's technology rapidly wore out. The Enterprise's starboard nacelle was severely affected by this as its intercoolers broke down, forcing Lieutenant Commander Scott to take it offline.
Though at first theorized to be just machines, when a disabled unit was brought into the Enterprise's shuttle bay for close range analysis, Commander Spock learned through a mind meld with it that the units were not pure mechanical. They were described by Commander Spock as "Neither machine nor animal." He determined them to act solely on instinct, though Doctor McCoy had tried to prove the opposite. They were also quite ancient, perhaps as old as the Milky Way Galaxy itself.
The units communicated with each other through "machine telepathy", allowing them to organize and deploy their numbers in formation types such as surrounding star in a sphere formation to use their chronometric radiation fields. Lieutenant Uhura was able to determine the frequency in which the units communicated with each other and the crew devised a strategy to use this connection between the units to disable them before they would age the main star in the Beta Reticuli system and destroy its two inhabited planets.
Taking a position within the unit's sphere formation, the Enterprise generated a jamming signal that disrupted the communication between the units. Once the units could no longer communicate with each other they shut down and turned adrift. Afterwards Commander Spock informed Captain Kirk that the units would eventually be caught by the star's gravity and be drawn in, destroying them completely. (TOS comic: "Swarm")