Using a shield means you have to keep track of two separate stats. The first is Block Rating, which is the percentage multiplier on the character window: it tells you the chance for you to block an attack against a mob at your level. The various Block Rating on gear like the Battleworn Tuskguard or Bulwark of the Amani Empire (both pictured to the right) add to your chance to block an incoming attack the same way that Crit rating adds to your chance to crit or Defense rating adds to your ultimate Defense score. You can even see that the nice folks at Wowhead have done the math for us on how the rating converts to chance to block. Block rating, however, only tells you half of the story.
If you look again at the helmet and shield I linked, you'll also see that they have a stat that reads "Equip: Increases the Block Value of your Shield by 51." This is known as Block Value, and it is the meat of the block mechanic. Block value is the amount of damage you can absorb on a typical shield block, unmodified by talents. Warriors have a talent called "Shield Mastery" that increases the damage absorbed by up to 30%, and the WoWWiki link takes you to the exact new math caused by this talent or the various Paladin or Shaman ones, but my eyes start to bleed when I look at formulas. At any rate, block value is important for both tanking classes that use shields because it tells how much damage the shield is absorbing on a block, and it's important for a warrior because Shield Slam, one of our big threat moves, is increased by the block value (not the rating) the character possesses.
So there you go: shields, and why I would raise a whole household of the little buggers if I could.