Currently, there are three competitive Druid archetypes in Hearthstone: Big Druid, Jade Druid, and Aggro Druid. While the first two actually require big, expensive cards, the Aggro deck can easily take a win by simply relying on cheap yet effective creatures.
Kobolds and Catacombs gave us Dire Mole, a super-cheap beast with 3 points of health, Plated Beetle, another low-cost beast that gives you armor, and Hoarding Dragon, a 4-mana, 5/6 minion that can either finish off the opponent on turn five or deal with a big taunt that stands in the way.
The rest of the cards are standard Druid buffing spells, such as Mark of the Lotus, Power of the Wild, and Savage Roar. Together these cards make up some really fast and aggressive decks.