A Wireless Event (Also called Wireless Distribution Event) is a special event which gives either a Pokémon or an item to somewhere special.
In Pokémon Diamond and Pearl, there's one that gives Darkrai.
To recieve item on special events, you must have A DS, the specified game, obtained Pokédex in that game and no more than a number of Wonder Cards. After it ends, it will not be coming back. The only way to get it is to have a friend who recieved it during the time.
Here's a list of Known Events:
Secret key: Unlock room to Rotom's room to get machines that can change Rotom's form.
Member Card: Unlock an area to catch Darkrai.
Celebi: Get Celebi
Get Victini: get Victini
Get Zorua: get Zorua (Require Celebi)
Get Zoroark: Get Zoroark