Spirit Caller is played the same way as all other Yu-Gi-Oh! games. You duel opponents with cards, either by summoning monsters to fight, using magic spells to assist your monsters, or by playing down traps. Each player has a total of 8000 "Life Points". A player's goal is to knock down their opponent's Life Points down to 0, which is done by attacking their opponent directly with their monsters. Decks are made up of 40-80 cards. At the game's beginning, you choose your avatar. You are allowed to pick a name, and customize between a certain amount of hairstyles and face expressions. Different outfits and Duel Disks can be won by repeated duels against certain characters
Source: Wikipedia, "Yu-Gi-Oh! World Championship 2008", available under the CC-BY-SA License.