Gear is huge for tanks. An undergeared one may not cut it despite being an amazing tank, and an overgeared one may be able to get by despite being a Failtank. As you go along, you should pick up different pieces of gear for different situations.
Obviously as you are first gearing yourself, some pieces are just going to be wonderful upgrades all around. Gearing for success really comes in once you have the ability to switch out a few pieces of the puzzle to make yourself a different kind of tank. I am going to go over some of the differnt sets of gear and what they are good for.
Effective Health Set: The ole EH set is the one that will give you the most stamina and armor so you can take bigger hits and survive. This is the set that will often be used while progression raiding. It really is pretty simple. Stack stamina, stamina, stamina. You will want to enchant for stamina and gem for it as well. Make sure you still keep your 540 defense (and your meta socket working).
Avoidance Set: This is the set that makes it so you potentially take less hits, although your health pool will be smaller. The priority stats in this set are defense, dodge, and parry. Defense Rating is a wonderful stat for avoidance as it provides dodge, parry, and even some block. You will want to stack Dodge and Defense until the diminishing returns starts to become too great. I would not recommend geming for parry ever. It is just not as effective as dodge or parry. The Avoidance set has a lot to do with the RNG. There will be strings of parry, parry, dodge, dodge, dodge, dodge from time to time. There will also be times when you get hit 5 times in a row. These moments is when having the smaller health pool is rough.
Boss Threat Set: If you are looking a maximizing threat, the two best stats to stack are expertise and hit. Expertise is better than hit until it is capped. Once you are expertised capped, start going for hit capped as well. After hit cap, strength is a very nice threat stat as it gives you block value and attack power. The reason the expertise and hit are so important is that misses, dodges, and parries by the boss do no damage. Hitting him every time will make your threat jump through the roof.
Block/Trash Set: Block Sets are exactly what they sound like. They maximize your block for some nice mitigation. The nice thing about block is that you can stack it all day long and not worry about diminishing returns. Block sets are often used for those big AoE trash pulls for a few reasons:
a) Blocking a lot of incoming damage makes it much easier on the healers.
b) Damage shield will proc A LOT, which will help out enormously with your AOE threat.
Gearing the right way for the right encounter is very important. It can mean life or death for the whole raid. Please tank responsibly.
Check out Veneretio at Tanking Tips for someone who does this much better than I can. He has some wonderful articles, and should be on top of any tanks browser list.