ArenaNet is glad you've asked and has prepared a well-researched essay on the topic. The Aetherblades are a new group that has crashed the Dragon's Bash and is looking to make all kinds of trouble as players head into Guild Wars 2's living story for July.
The devs struggled with how to make a new pirate faction that wasn't quite like all of the other pirates in an already piratetastic title. The solution, the article says, was to combine steampunk, lightning, and airships to make a dashing group of electric buccaneers: "We imbued the Aetherblades a lightning aesthetic: They arrive in a bolt of lightning and their skill effects, traps, and weapons are all laced with electrical effects."
The article also outlines some of the mobs players will face when they tangle with the Aetherblades, from the basic minions to the razzle-dazzle officer corps.