Halo 3: ODST has flashbacks. That, we knew. You're a rookie ODST troop, tasked with tracking down some lost squadmates.
While you're looking for them, the game is a little bit open-world. Once you find those clues, the game switches things up; you get a flashback of what happened to that soldier, and then you play that flashback.
In the latest issue of game magazine GamesTM, Bungie's Joseph Staten states that each squad mate has a "very distinct personality" — i.e., they have names, faces, etc.
"The reason we decided to keep the rookie faceless and anonymous is to help smooth these clue-triggered transitions from one ODST to another," says Staten. "[The game] is designed to put you in the boots of a bunch of fun, memorable characters so you can experience this awesome pocket of Halo fiction from as many points of view as possible."
Thanks shaunomacx for the tip!