Druid spells are tied to wisdom, which determines what level of spell the Druid can cast, how many of each spell, and what the spell's DC will be.

For the spirit shaman, who also use the Druid spell list, wisdom modifier is used for determining the spell level available and how many per level, while charisma modifier is applied to the DC of the spell.

Druid/spirit shaman spells are divine, so they are not affected by somatic spell failure caused by wearing heavier armors.