Small spiders mutated by magic, they always come in large numbers.
Spiderbroods have all traits commonly shared by spiders: fast, individually weak, resistant to magic and extremely numerous, even more so than other spiders. Druids are essential to deal with Spiderbroods, as they come in swarms so large that barracks, mages and even archers are ineffective.
The first few groups of Spiderbroods can be countered by Sentinel Arbors or Arcane Archers, however later waves require you to build Druids. A Weirdwood can instantly destroy whole swarms with tier two Clobber, while the Arch-Druid Henge can constantly bombard them with stones, and applying the Sylvan Curse to deal massive damage to the entire group.