Nintendo WiFi Connection
From Wild World Wiki
The Nintendo WiFi Connection is Nintendo's free online service. It allows you to play WiFi games against players from around the world. Animal Crossing: Wild World makes great use of this by letting players from around the world visit each other's towns, chat, and send mail to each other among other things. The Nintendo WiFi Connection is also known as the WFC, NWFC, and WiFi.
Getting Set Up
All you need to go online in AC:WW is a compatible wireless hotspot (for more information on making sure your WiFi is compatible, go to the Nintendo WiFi Connection Support Page, where full details can be found). To go online, you must visit Copper at your town Gate. The first time you go online, he will issue you with a Friend Code so you can start collecting friends. From this point, to play over WiFi with a friend, you will need your friend's friend code, name, and town name entered into your DS (enter them on the Friend Screen.
To allow your friends into your town, you must go back to Copper and ask him to open your gate. This will allow your friends to enter your town. When a friend arrives, an icon will flash on your screen informing you of this and then the screen will swap to show your friend arriving in your town gate.
To go to another friends town, you must also visit Copper. Tell him you want to go out, and he will activate the NWFC. He will then present you with a list of all your friends who have their gates open. Select the town you want to visit and confirm you want to go there. After a brief data exchange period you will arrive in your friend's towns Gate.