Mastery is a hot topic all over the wow blogs and websites at the moment. After reading a series of threads, and a lot of player conjecture – here is my quick definition:
Mastery: will be a passive increase to your current form/stance/spec’s role. Do not expect it to define the specs, expect it to enhance it. It will be present on level 81+ gear, will change when your role does, and is still very much being worked out.
TL;DR = It’s a passive buff, subject to change; so worry about it 1-2 weeks after Cataclysm expansion is released, when the Theory-crafters have sussed it.
The wow.com one contains back links to all their class and spec related posts, and all sorts of goodies which are also moot at the moment.