Shadow Priest and You

Ok, since I'm sick of seeing people spec their Priests as shadow then failing to dps anything, I'm going to teach you all a lesson. A lot of this stuff crosses over to every casting class, so you stupid Mages and Warlocks can pay attention too.
Talents
This is probably the easiest part, since only 2 trees have viable talents for shadow: Early disc and full shadow. I recommend the 14/0/51 +6 build, and I like to spend the extra 6 points in Improved Psychic Scream, Silence, Psychic Horror, and Improved Shadowform. Kinda pvp-ish, but when you look at the other talents available, you really are limited.
Stats/Gear
This is really half of what makes a Shadow Priest viable. First thing you need to do is become hit-capped, which is 17% hit in a PvE environment. Seems like a lot, but remember that Shadow Focus and Misery give you 6% hit already, so you only need 11% more. 289 hit rating will ensure you never miss (except on Magic, that number still seems a bit wonky). After that, hit rating is useless so get as much SP as you can.
When gemming, the easiest thing to do is go +SP in every socket except when you either need to meet a meta gem requirement (CSD is the best, and only needs 2 blues), or when there is +5 SP or +8 crit (ish) bonus for socketing.
Between haste and crit, get your haste first up high enough so that you can Mind Flay twice in 5.5 seconds (Mind Blast cooldown), which is around 320ish haste. After that, Crit all the way. In 3.3, haste will affect DoTs, but until that glorious time, we gotta go with RNG procs instead.
For glyphs, there are really only 2 decent ones: Glyph of Shadow and Glyph of Mind Flay.
Rotation
This is the second half of what makes a real Shadow Priest. While there is no set rotation (in WotLK, no classes have one, save for a few), there is always a priority system, which goes something like this:
1. Vamp. Touch
2. Devouring Plague
3. Mind Blast
4. Mind Flay
Once VT and DP are up, Mind Blast. When it's on cooldown, refresh DoTs only if they need it (clipping them restarts their ticks, which is a loss of 1 tick. Better to be late than early here). If they don't, Mind Flay once. If they still don't, Mind Flay again. Since you have enough haste for 2 of em, Mind Blast is off cooldown, so use it again. Only clip Mind Flay if there is more than 1 second left in it's duration, and only clip it for Mind Blast. Sometimes you'll find yourself sitting around doing nothing for a second, but that's the way it is.
Notice how Shadow Word: Pain isn't on the list? This is because you should only ever cast it once: when you have 5/5 Shadow Weaving, and every trinket has procced. Some people even pop Wild Magic Potions just to get a higher effect out of it. Why? Shadow Word: Pain is refreshed by Mind Flay, and will continue to tick away at the Spell Power and Crit chance at which it was cast. Only refresh it manually when you're going to make it better.
DoTs are ahead of MB on the list because they have a 1.5 second cast time, and 15/24 second damaging component. You'll actually do more damage casting them earlier than later, so Mind Blast can wait. I personally hate Shadow Word: Death in a rotation, because it's about the same as 2 Mind Flay ticks, and with Mind Flay able to crit, Shadow Word: Death just doesn't seem worth it.
For the love of christ, DO NOT USE MIND SEAR. It does absolutely NO DAMAGE to single targets. When people use it, you are allowed to openly /laugh at them. Hell, make sure everyone in the raid knows how retarded that priest is, and get him kicked. He'll probably roll plate gear anyways. It's only good for more than 3 targets, and you should be DoTing the one you cast it on!
Mana
Last but not least is mana. You can only dps hard so long before you dry up, but being a Shadow Priest you have a few tricks before that happens. First is Shadow Fiend. It is an increase in dps (think of it as another DoT) with a 3min CD that gives you mana. Use it when you're around 66% mana and you'll go back to full without overflow. Or be tricky like me and use it on XT's heart phase, etc, and watch as you pass the weaker rogues, warrs, nan, palas, eldes, and dumbknights on the meter. The other, Dispersion, actually takes you out of combat for 6 seconds to give you 36% total mana. Best used when moving around (between IC members, for example), or when you're taking heavy raid damage you can't shadowheal through (Mimiron, maybe).
In Conclusion
Make sure there is a Shadow Priest in your raid: He gives Replenishment, as well as 3% to hit for everyone. Duel wielding classes will pee their pants in excitement. Make sure all DKs who use Obliterate are talented to not eat Plagues, otherwise you'll have a stroke over the number of times you'll be reapplying your 24 second DoT. And if you see a non hit-capped Shadow Priest in your raid, cry like a girl on the last day of camp, and make sure the leader kicks them or makes them heal. Nothing worse than having your SW:P overwritten by someone who will miss every other spell. He fails.
Sorry for the wall of text (it crits harder than Demo's Obliterate, so watch out).

Crazy, please add here for Shadow pvp and for Disc pvp if u can. Kthnxbai.

Nice guide for shadow thanks!

nice guide i want to mention few things here

there is a glyph called glyph of shadow please do add that to shadow priest i know i am noob at playing priest its just an inpiration but we