Some terminology

102.4:

The holy grail of shield tanks. A boss level opponent (during this expansion usually level 88) has you needing 102.4% CTC (see below) to remove unmodified hits from the possible outcomes of a physical attack.


Avoidance:

It's the sum of miss, dodge and parry. In percent. There's an inherent 5% miss built into the game, and you can find your dodge and parry percentage by looking at your own character interface. Very often players tend to add block into this number.

How do you know? Well, if they quote an avoidance number above 50%, then they have added block.


CTC:

Combat Table Coverage. It's simply avoidance plus block. Again in percentage.


MDR:

Mean Damage Reduction. You won't encounter this acronym as often as CTC (or a wrongly used use of the term avoidance for that matter). It stands for how much physical damage you'll avoid taking statistically. Maximizing MDR is not the same as maximizing CTC.


Pull:

Casually means the very moment when you (or someone else) activates the opponent, usually by attacking.
In raids the term pull very often includes the first thirty second of the fight. The reason is that this is the period where a tank risks losing aggro to someone else.


DD:

Damage Dealer. Often called DPS. Any member of the raid/party who's not a healer or  a tank.


RDPS:

Raid Damage Per Second. The total sum of all dps. If a boss has an enrage timer, then you can easily calculate the RDPS needed to down it in time. From that number you can estimate the dps needed for every DD involved. Observe that ths number may not be sufficient in reality. There might be parts of the fight where hammering away at the boss is impossible for one reason or another.