In Plants vs. Zombies: Retro Attack , there are level groups called Worlds.
The first world is World 1: Peashooter. It's like a tutorial world in the game, teaching in hands-on what would have been read in the game guide.
The second world is World 2: Teashooter. This world introduces the player to Teashooter, a PvZRA-exclusive plant which shoots stronger peas.
The third world is World 3: Repeater. It has 9 levels, unlike the 6 in the other worlds. Unlike other worlds, World 3: Repeater was released in parts. This was mostly done to get it out quickly and efficiently.
The fourth world is World 4: Gatling Pea. It has 16 levels, and the 16th level has the most powerful boss ever. In this world, you encounter TWO new zombies: The Helmet Zombie, which has mega HP, and the Gatling Zombie, which shoots at your Peashooting Plants from afar.
The fifth world is in development.