Ten Ton Hammer has published the fourth in its series of "class interviews" with Funcom. Previously it spoke with the Age of Conan devs about the Dark Templar, the Guardian, and the Conqueror. This time it's the Assassin.
You can read the interview for all the details, but the gist of it is: Asassins use stealth and quick bursts of high damage melee attacks in tandem with poison and debuffs to fell opponents quickly. As a counter-balance to their extraordinary offensive capabilities, they are very weak in terms of defenses. They wear thin silk armor because it allows them to move freely and silently.
So the class is about 90% the same as the Rogue class in games like World of Warcraft. Unfortunately, "Assassins do it from behind" just doesn't have the same ring to it.