Left - Union Room, congregate and trade with people in the vicinity of your DS. They do not have to be friends. Doesn't use Wifi at all.
Center - Wifi. Must be on someone's friend list to appear to them here. You likewise can have up to 32 people here with their FCs as well.
Right - Global Link. Drop off a Pokémon in exchange for one someone else has already dropped off, for instance.