I don't like the Tennessee Volunteers (Phil Fulmer screwed Michigan in 1997, I wish Eric Berry went to Michigan, I don't like orange sherbet), but Rocky Top Talk has a good breakdown of zone blocking. I agree with the majority of the post . . . except the explanation of the difference between zone blocking and man blocking. It's just not explained well enough.
There's no blocking system out there in which a play calls for an offensive lineman to block a particular defender; "man" blocking is a misnomer. A wham/iso play doesn't require the "offensive tackle to block the defensive end" because what if there's no defensive end? What if the defensive end is lined up over the guard and a strong safety is walked up and playing as an edge rusher?
Zone blocking asks the offensive linemen to create a screen. For those of you who have played intramural flag football, some leagues ask you to "shadow block" (i.e. no contact); that's essentially what zone blocking is. Zone coaches want their offensive linemen to get in the way of a defender and then let the running back choose whichever hole has the fewest obstructions.
Regardless, this is an excellent post by Rocky Top Talk. It's not entirely applicable to Michigan's blocking scheme because it's mainly talking about running the zone play from a single-back set with the quarterback under center, but most of the basics still hold true.
*Unfortunately, I've given this bit of advice to several players who suddenly forget their plays when it's game time.