From the publisher's website:

In Smash Up players take two factions, such as pirates, ninja, robots, zombies, and more, and combine their decks into a force to be reckoned with! When the game begins, players take turns drafting one of several faction decks. Once each player has two decks, they shuffle them together, combining their distinct abilities into a unique 40-card deck. Players then deal out a number of Base cards, and play their decks to attempt to have the most power on a base when it is scored. It may sound simple, but how simple is it when your opponent’s Zombie-Ninja Assassinate your minions then create an Outbreak, loading the Base with minions from their discard pile? Or when their Pirate-Dinosaurs Full Sail onto the base with the mighty King Rex, whose massive power dwarfs that of other minions? Each combination gives a different experience, Pirate-Aliens play very different than Pirate-Zombies, and with 8 factions in the base set there are tons of combinations to try out!