The term "Swarm" in TSW refers to the difficulty rating that a monster can have, indicated by a three-small-dot icon next to it's name. Note: The dots will be replaced by a three-small-flag icon if the monster is a mission objective.
"Swarm" monsters differ from other monsters in that the swam is comprised of multiple individual monsters, typically 3-4, although can range from 2-5. Each individual within the swarm is considerably weaker than non-swarm, normal-difficulty, monsters in the same area. However, it is expected that the swarm as a whole will be of comparable difficulty to a normal monster.
All members of the swarm share an aggro-triggering mechanism - and you cannot pull one member of the swarm without all of them attacking together. However, they do NOT share hate calculations, and it is possible for one member of the swarm to be attacking one player while another member of the swarm is attacking a different player.