Though the gameplay is largely the same as that of Ruby and Sapphire, Emerald introduces new features. The plot is modified; both Team Magma and Aqua are villains who manage to awaken Groudon and Kyogre respectively. When the two legendary Pokémon begin to battle each other, the protagonist must unleash Rayquaza (pictured on the box cover) to calm them. Some of the game mechanics are changed as well. Though in Ruby and Sapphire, double battles were clearly marked, in Emerald, they are more sporadic; two separate trainers might come together to battle as a pair. After the Elite Four is defeated, the player may re-battle Gym Leaders in a double battle. Also, Pokémon sprites are animated in battle, similar to that of Pokémon Crystal.Probably the most significant addition is the Battle Frontier, an expanded version of the Battle Tower present in Ruby and Sapphire.

(Wikipedia)