The High Link is a new feature within Pokémon Black & White which allows you to connect to your friends and walk around the various parts of Isshu. This feature is done over local wireless. This mode is not connectable via the Pokémon Centres but in a special area in Isshu. Here, when you're hosting the High Link, everything looks normal. However, when you're the one joining somebody else's High Link, you enter a version of Isshu that is in monochrome and appear as the multiplayer avatar of your game character.


In the High Link, you can walk around most areas and find your friend. You also get to find numerous support missions for you to complete to get prizes. You can team up with your friends to do these missions and other missions require you to face up against your friends.
It is currently unknown how many missions there are to be able to use or any other extra features brought in.

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Info taken from Serebii

This seems like it has the potential to be really cool.