Hey Anti :)
The Druid is one of my favorite classes (probably my favorite class period).
In traditional D&D you can find that they cast spells in addition to shapeshifting, although that changes a bit in certain sources and builds. One of the awesome things about the druid in DW is that they aren't casters by default, but the designers are sneaky ninjas insofar as they built in the possibility of a build that makes them closer to the D&D vision. How?
If you take Hunter's Brother or Stalker's Sister - you can gain a Ranger move. This opens combos to you. So for example: you can take 'God Amidst the Wastes' which gives you access to the full cleric spell list. Boom - casty Druid.
However, while the cleric spells are fine, they are not necessarily all classically druid flavored. If you compare this list to the cleric list you'll see that I translated it spell for spell and mechanic for mechanic. This isn't a new rule so much as a re-skinning for those Druids that want to play a casting heavy cleric-y type druid.
They keep their shapeshifting and all moves as-is, this is just a flavorful supplement to let those casters have more classically named and themed spells in their list.