The High Link (Entralink in English versions) is an area that you access via the C-Gear.
High Link Forest
When you connect to the Dream World, either waking a Pokemon or putting one to bed, you will obtain all the Pokemon that had been set in the PGL to go to your game. They will appear in the High Link Forest for you to catch. When you battle a Pokemon there, you will also get a new ball, the Dream Ball, which has 100% catch rate.
When in range for local Wi-Fi, you can use the High Link to connect to and visit other people's games. There are two main things you can do: Missions and White Forest/Black City restoration.
High Link Tree
White Forest/Black City restoration
When connected to another game this way, you can go to their White Forest/Black City and invite some of it's inhabitants to your own White Forest/Black City. This is important, because as time passes in your game, people will leave your Forest/City, thus reducing what you can do there.
- White Forest has more grassy/water areas, more/bigger trees, more/new Pokemon, items (0-3 each day) appear.