Swarms are seething masses of small creatures, such as rats, snakes or insects, summoned to the battlefield by magical means and set loose upon the enemy. Individually, the critters in a swarm are little threat, but their sheer weight of numbers more than compensates for their size. Swarms are made of a number of bases. Each base includes many creatures, but counts as a single model with a profile like the one below:

Skirmishers

Swarms fight in a loose and shifting formation, achieving through animal instinct what other troops achieve through rigorous training. Swarms have the Skirmishers special rule.

Unbreakable

Swarms are utterly unthinking, and completely devoid of self-awareness – they have the Unbreakable special rule.

Squish!

Whilst swarms are particularly nasty foes if the fight goes their way, it's easy for the enemy to inflict plenty of damage on the swarm once they've built up momentum. Most creatures in a swarm don't tend to die to weapon strikes, but are crushed underfoot once the fight goes against them. Swarms have the Unstable special rule.

Note that swarms who are subject to special combat result rules (such as Undead and Daemon swarms) do not have the Unbreakable or Squish! rules, but use their army's own special rules instead.