What’s the difference between Dirty Bomb and Extraction?
In short, the game's the same. Fun fact: Extraction was Dirty Bomb’s original name and the reason we didn’t stick with it for the Closed Alpha was that we weren’t sure if we could secure the worldwide trademark for it. We're confident we can achieve this with Nexon's help, so we’ve decided to go with the name we’ve always wanted for the game.

Official site: Homepage

More info can be found here:
Blog About Extraction.