If an item has +12 Defense Rating.....how and what specifically does that do for us?
What is defense rating?

At level 70, every 2.4 Defense Rating points equates to 1 Defense Skill (the number shown on your character inventory window).

Each point of +Defense skill adds 0.04% to the chance to be Missed, to Parry, to Dodge, and to Block for players. Mobs receive 0.1% per point of defense.

This means +25 Defense will grant you an extra 1% Miss, Parry, Dodge, and Block. The formula is constant across all classes.

Defense (up to your unbuffed level maximum) also decreases the chance of being affected by a crushing blow by higher level mobs. It further decreases the chance of receiving critical hits from any level attacker by 0.04% per point that the target's Defense skill exceeds the attacker's Weapon Skill. Mobs get an additional benefit from defense of Glancing Blows versus players.

However, all of these chances are based on an opponent of equal level. The rating difference between the attacker's weapon skill and the defender's defense factors into the true chance for miss, parry, dodge, block, crushing blow, glancing blow, and critical hit.

http://www.wowwiki.com/Formulas:Defense

With a little math, you'll see that at level 70, 60 defense rating = 1% those things, so your 12 defense rating = 0.2% dodge, miss, and anti-crit.

* 1 Defense skill = 2.4 Defense Rating (at level 70)
* Each point of Defense skill for Druids adds 0.04% to the chance to be Missed, to Dodge an attack, and to convert a critical strike into a regular strike. This means +25 defense skill (+60 defense rating) will grant you an extra +1% Dodge, non-player opponents -1% to Hit, and will reduce your opponent's chance to crit you by 1% (it will be converted to a normal strike instead). Warriors and Paladins gain additional benefits from defense skill in the form of +parry and +block at the same 0.04% rate.
* For a level 70 player with 350 defense against a level 70 mob (level*5 = 350 weapon skill), the mob has a 5% chance to critically hit the player. With the Survival of the Fittest talent, Druids need only an extra +65 defense (156 rating) to negate all critical hits ("crit immunity"). In other words a total of 365 defense at level 60 or 415 at level 70. There is always a rare chance to be crit even with maximum defense.
* As long as your base defense is maxed (350 for level 70), you've done everything you can do to avoid crushing blows. A level 70 player therefore cannot apply more than 350 points of defense against crushing blows versus level 71 and higher mobs, even if the player's modified defense is higher than 350. A crushing blow is an attack made by an NPC that is 150% of the damage caused by a normal successful attack. A player cannot perform a crushing blow.

So what do you need to be uncritible against a raid boss? They function as your level + 3, so how much more defense is needed to compensate for that?

The magic number is 415. So you need +65 defense (with Survival of the Fittest 3/3), which is actually like +165 defense rating.

Please look up at gabrielk's post for more info.

Ah, after looking at the math, I understand now. The 415 includes the extra 0.6% that the bosses get.