If you have already read a reliable review on Ruby/Sapphire then what I will say you have already heard. Emerald is basically the same exact game as Ruby/Sapphire with a little extra content but nothing that makes or breaks a game.
Like every other pokemon game the objective is to catch em all, train em all, and defeat all the other trainers. You start the adventure as a enthusiastic young trainer who wants to be a pokemon master. With that in mind he(or she if you want)sets out to realize his goal. The rules of battle remain the same, put your opponents pokemons HP to zero before they do likewise to yours. Defeating a gym leader earns you a badge and eight is needed for the opportunity to challenge even more powerful trainers. Defeat the more powerful trainers and you gain the right to play the most powerful trainer. Beat him and you have beaten the game. Simple.
After ten years I was hoping for something a little more inventive. A different place to explore would have been welcome. Whatever, the game will still sell to seven year olds right?