GTS

From the Pokémon Wiki, a database for the Pokémon series that anyone can contribute to

GTS, or 'Global Trade Station' is one of Pokémon Diamond/Pearl's key features. using Wi-Fi, it allows people from all over the world to trade their Pokemon via putting their Pokemon up on offer or finding an offer and accepting it.

The GTS has come under fire though, seeing how during July, nintendo stopped the trading of legendary Pokémon.