Pokemon Emerald is the fifth game in the hugely successful third generation of Pokemon games. Similar to Pokemon Ruby & Sapphire in style and gameplay, it has a few differences in it that make the game even more interesting.
Difference 1: Different storyline. Well, same basic storyline - teenager goes out with his/her first monster and tries to catch all 200 while at the same time saving the world. The difference is that (s)he's got the task of stopping both Team Magma and Aqua, evil gangs who try to turn the world into either a big ocean or a big patch of earth.
Difference 2: Wireless adapter compatibility. Introduced with Pokemon LeafGreen & FireRed, these tools allow many people to communicate through their gameboys without cables or wires. All the features that come with the adapter are added into the game too.
Difference 3: Other minor differences like upgrades to items in the game and colour of clothing. Gives the game a sort of "green" feel.
Difference 4: Entrance to the Battle Frontier. The biggest place to battle in a Pokemon game. The Frontier offers challenges never before seen, and is the main idea behind a mysterious man in sunglasses you meet during the game. The Frontier is the best part of Emerald and arguably the one where people save their games the most.
The Battle Frontier offers seven challenges in seven buildings: an arena, a dome, a factory, a tube shaped like a snake Pokemon, a palace, a pyramid and a tower. Each offers their own unique style of fighting with Pokemon and their own strange rules. One stops you from choosing heaps and heaps of healing items, one stops you from choosing your Pokemon's attacks, and one even stops you from using the Pokemon you caught, raised and maybe even bred. (But it's still quite fun.)
So now I've told you the basics, it only remains for me to say: go out and get a copy!