Mastery is a secondary (like haste, crit, dodge etc.) stat that can be found only on gear (you can't put any points in it anywhere).
Even if there are items with mastery, that are usable below lvl 80, you can learn mastery skill from your class trainer at lvl 80. This skill actually allows you to benefit from mastery stat on gear (you can probably find some Cataclysm greens usable from lvl 78 that have mastery, but untill you get mastery skill they are as good as if they were not there).
You can see detailed mastery description by pressing 'N' and clicking 'View summary' button. Each class has 3 specs, and each spec has different mastery. Your mastery will be the one from your main spec tree you've chosen (for example: you play retribution paladin that has some points also in holy tree, but your mastery will be Hand of Light - retribution tree mastery)
Generally mastery is not some secretly awesome god-mode. It's just yet another stat, the difference being it can only be used from lvl 80.
It's importance is different for different classes, so you should check http://www.mmo-champion.com or http://elitistjerks.com for advice on if it's good for certain class/spec.