I'm hearing some conflicting things about this game.
From what I've heard, it's played out on one HUGE map, divided into 30 sectors. Stuff will keep happening in other sectors while you try to traverse through it looking for your objectives.
It seems to be aiming for that wide-openess feeling that was lacking in HL2, Halo2, Doom3. And yeah the backgrounds are gorgeous (though the in game models looked hideous from what I saw of it last time).
Multiplayer worked the same way I think, several "STALKERS" load up and play on one huge map, challenging each other to reach certain goals or something like that.
Originally there was talk of no load time between sectors(?) or they were aiming for a seamless giant map or something, but then the last preview I read said there was in fact load times between sectors and so forth.
None the less, it sounds fairly interesting.