So here is a nice Texans blog attempting to explain Zone-Blocking system for the Huston fans that aren't familiar with it. Nice read, but rather long.
He has two posts on it, first post:
http://www.atexansblog.com/2007/11/28/professor-without-knowing-precisely-what-the-danger-is-would-y ou-say-its-time-for-our-viewers-to-crack-each-others-heads-open-and-feast-on-the-goo-inside/
Second post:
http://www.atexansblog.com/2008/01/11/soabout-that-zone-blocking/
And for those who really want to read more, here is Football outsiders take on ZB system:
http://www.footballoutsiders.com/2005/03/31/ramblings/stat-analysis/2484/
Cheers!