The Pokemon Global Link is part of the main Website after Black and White were released. Consisting of three main things to do. Use the Dream World, Enter Wi-Fi competitions, and receive Promotional Gifts.
Dream World: The Dream World involves using Wi-Fi connection. In B/W you helped Fennel obtain the nesseccary requirements to finish her research. Which was to make Pokemon dreams come true. When that was finished you gained the C-Gear with the connections of Online, Wireless, and Infered. The online and wireless setting is what you use to transfer to the Dream World. It asks you to connect to Wi-Fi connection and select a Pokemon to go into the DW. You will need a profile in Global link to officially obtain DW Pokemon. When over with that you have an hour in the DW to catch Pokemon, grow berries, and explore there. You also need to have required a 12 digit code on the main screen of your DS that allows you to send/receive Pokemon, enter the competions, and obtain any promotions.
Wi-Fi Competitions: As stated in the title it needs Wi-Fi to be entered. You enter one with a team in your battle box and a digital player ID. They hold different kinds of tournaments just like here with restrictions such as in a single battle through Wi-Fi. You can't use certain legendary Pokemon, the Soul Dew is banned, three Pokemon for one battle though you can have a team of six in your battle box, and certain moves are banned.
Promotional Gifts: Requires you to have a account on the main website and the global link. Certain gifts you receive are for the Dream World, event Pokemon or berries. Some gifts don't require to have a password so it's like a free gift. On your b-day you get a special Pokemon with its DW ability. Some Pokemon events can only be obtained with a certain code such as the DW starters giveaway.
You can also edit your profile, check your Dream points and see what past Wi-Fi competions you have entered.