Pokemon XD is a sequel to Pokemon Colosseum for Gamecube. It has an RPG mode and a Battle mode, much like Colosseum. You can also catch a few select wild Pokemon in the game.
There are two mini-game type elements in Pokemon XD. One is the Battle Disk where you use Disks that you collect throughout the game in order to simulate battles with certain conditions. The other is called Battle Bingo and allows you to battle with the fourth generation Pokemon, Bonsly.
You'll also be able to catch Shadow Lugia, purify it and trade it to your Ruby, Sapphire, Emerald, FireRed or LeafGreen game.