Druids are a creature type primarily associated with green mana, lands, and mana production. Many Druid decks use their creatures as an expanding mana engine before turning that abundance of resources into a much larger threat.
Priest of Titania and Elvish Archdruid are both Druids capable of producing significant mana in Elf-heavy decks, while Beast Whisperer brings card draw to a creature type already overflowing with ramp.