Vindictus is a pretty killer game. I've been playing it since Closed Beta and absolutely love it. its more action oriented than most MMOs out there, if I would have to choose a game similar it would have to be the Monster Hunter series.
The story of the game is about a prophecy that says if the Humans spill all of the Fomor's (the enemies) blood, the Goddess will descend to earth and deliver everyone to paradise. You start out as a mercenary for the local town's militia and slowly rise the ranks into the Royal army and uncover more of the story. Of course there are twists and turns and everything in between that keep it really entertaining.
As far as what you actually do, you slay beasts of pretty much every variety in instanced dungeons. Normal dungeons have a limit of 4 player parties while the "Raid" dungeons can have anywhere from 6-24 man epic battles. If you wanna see some high level action I suggest watching the episode 8 trailer and the episode 10 trailer.
http://www.youtube.com/watch?v=XqH0_Zu4Sbk