According to the latest issue of Game Informer, Halo 3 Recon will have an open-world environment to explore.  Your character is a faceless orbital drop shock trooper who was part of a five man team now lost in New Mombasa.  You will be able to explore the city in an attempt to find your lost comrades.  Upon finding the last known location of each of your teammates you will be put into a flashback sequence and relive what that ODST has experienced through a more linear gameplay style.

Of course playing as an ODST is going to be much unlike Master Chief.  Your enemies may seem the same but you will not have the pure physical ability and size that Master Chief had.  This will offer a completely different gameplay experience not seen in any other Halo game.

[Via CVG]