In one of the videos it a few of the bungie workers say that it's an FPS but it sounds like more of an MMO. If they are the same and I'm just being stupid, please forgive me :/
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... An MMO is a Massively Multiplayer Online, and an FPS is a First Person Shooter. So yes you can have an MMO while being a FPS, for example PlanetSide 2.