What's it about? Well. It's a multiplayer-focused RPG/ simulation game. In the action mode, you're teaming up with up to three other players to challenge some bosses (which does indeed include dragons) to collect monster cards and other assorted bits of loot.
That loot helps out when you head into the simulation mode and start building up your city. There you can use the monster cards to summon the monster army to attack rival cities or form alliance with others.
Sounds like a hardcore Farmville, then. Awesome.