Pokémon Emerald is the third game set in the land of Hoenn. On first looks this game seems to be identical to it's predecessors, Ruby & Sapphire, however there is much more that meet's the eye in this game
First thing you'll probably notice are the Pokémon's Animations, similar to how they were animated in Crystal, which are making a welcome return to the Pokémon games. Also, the graphics in this game are sharper than in Ruby/Sapphire with more detail to the overworld
Both Team Aqua & Team Magma are heavily involved in this game's storyline as they both try to awaken their respective Pokémons...and when they do, thats where the game radically changes from the other games with a whole bit where they battle and you have to stop them.
The Battle Frontier contains 7 buildings which all incorporate a different style of battling where you have to battle through multiple trainers to get to their leader's, The Frontier Brains. These battle styles include Rental Pokémon, Random Rooms, Relying on your Pokémon's Nature to battle, Tournements and many more...definitely a way to challenge any trainer
The Gym Leaders & Elite Four have also been updated with different Pokémon and a whole new gym leader! To make things even better you can rebattle the Gym Leaders up to 4 times via the Match Call feature