First-tier rights are for football and/or basketball games broadcast nationally.
Second-tier rights are for football and/or basketball games not selected by the first-tier rights holder.
Third-tier rights are any games not selected by the first- or second-tier rights holders and rights for all sports other than football and basketball. These rights are often sold on a per-school basis (not negotiated by the conference as a whole) and often go to regional networks (Comcast Sports Southeast, Raycom, or SportsNet New York, for example). They can also be reserved for networks like the Big Ten Network and the Texas Longhorn Network.
Deals are now being done for multiple tiers, though. For example, the Pac-12's new deal with ESPN and FOX covers first- and second-tier rights. And the ACC's deal covers football, men's and women's basketball, Olympic sports and all conference championship games. Basically, it's an all-inclusive package with a sublicensing arrangement in place with Raycom for games not broadcast by ESPN.
http://espn.go.com/blog/playbook/dollars/post/_/id/705/