The Global Link consists of two main features. The first, Pokemon Dream World, lets you put their critters to bed in the DS game, which syncs it to the site. Then, you can enter the dream online to customize the Pokemon's home, trade items with other players, and play mini-games. Once you're done, the progress syncs back into Black and White with all the items and new Pokemon you've found.
The second, the Global Battle Union, lets you compare your battle record to other players. Top players may be invited to qualifying events in national Pokemon tournaments. It also includes the Global Trade Station to track Pokemon trades. Oddly, the press release doesn't seem to indicate the Trade Station would itself facilitate trades.
A time wasn't set for these features, but the press release notes it will be sometime after the game's launch in March. The Global Trade Station is set at "later this year," so that feature may not be available when the site itself goes up.