We have a blog entry on The Wire that dives into the nuances of each division. The main thing to know is that the NCAA governs all divisions (except for the NAIA) and D-I is split into two separate entities known as FCS and FBS. Typically, the FBS schools are much larger in regard to the student population and athletic budget. They are composed of the Group of Five and Power Five conferences. FBS teams compete in bowl games in the postseason. The FCS, while still, D-I is lower on the totem pole regarding skill level, for the most part. They also compete in an entirely separate postseason that is more similar to a playoff bracket like basketball’s March Madness.
Of course, outside of D-I, there are D-II and D-III levels that vary in regard to their layouts, scholarships, and postseasons as well.