As a contrary to the holy tree, which focuses on the heals, priest discipline tree has its emphasis on survival and damage reduction. Most priests that spend their points in the Discipline tree are doing so to be PvP healers.
Tier 1
Unbreakable Will: Max 5 points, up to 15% chance to resist stun, fear and silence effects
Mainly a PvP talent although it has its uses in some PvE content, extremely useful in PvP though
Recommended for all PvP players
Twin Disciplines: Max 5 points, Increases the damage and healing done by your instant spells by 5%.
A nice all around talent. It allows your quick spells to do additional damage or healing.
Great for shadow priests as your Shadow Word: Pain and Death are both instants.
Tier 2
Silent Resolve: Max 5 points, reduces threat generated by holy and disciple spells by up to 20% and reduces the chance your spells will be dispelled by up to 30%.
Excellent talent for healers to reduce the chance of drawing healing aggro.
Of some value in PvP so that spells can not be removed as easily
Improved Inner Fire: Max 3 points, increases the armour provided by inner fire by 15% and increases the total number of charges by up to 12.
Inner Fire is used mainly when leveling or when PvPing to prevent damage. The 15% extra armour is not a significant boost.
The 12 additional charges allow you to cast it less often thereby saving mana.
Skip it unless you are leveling or PvP
Improved Power Word: Fortitude: Max 2 points, adds up to 30% health to your fortitude abilities.
While 30% sounds like a lot, it isn't even at max level. Because it is not a lot of health you can skip it unless you raid.
If you raid then one priest in the raid will be expected to have this ability so that the group has as much health as possible.
Martyrdom: Max 2 points, provides up to a 100% chance to gain focused casting for 6 seconds after being critically hit. . The Focused Casting effect reduces the pushback suffered from damaging attacks while casting Priest spells and decreases the duration of Interrupt effects by 20%.
Very useful in PvP where you are going to be a prime target and can be critically hit easily.
Combined with other pushback prevention abilities it can almost remove pushbacks from your spells.
Recommended for PvP
Tier 3
Improved Power Word: Shield: Max 3 points, adds up to 15% damage absorption to your shield.
While this sounds like a significant amount it really isn't that much on your shield.
Unless you plan on taking Improved shield or aegis later on in this tree it is not worth the points.
Can be skipped
Meditation: Max 3 points, Allows up to 30% of your mana regeneration to continue while casting
Very useful for long fights both for damage and healing. Becomes more useful the longer the fights you are in become. Therefore almost critical to have for raiding or arenas. In these situations there are fights that last a very long time.
Inner Focus: 1 point, reduces the mana cost of your next spell by 100% and increases its critical effect chance by 25%. 3 minute cooldown.
Amazing ability! Any spell you want for free, and does not start to global cooldown or the 5 second timer and greatly increases the chance of a critical.
Can be used in macros with your favourite spell.
Insanely useful for both healing (holy) and DPS (shadow)
Tier 4
Absolution: Max 3 points, up to 15% reduction in mana cost for dispel magic, cure disease, abolish disease and mass dispel.
Nice when you need to use these abilities a lot, as in certain raids or PvP, but a heavy investment in talent points for a low return in most situations.
Situationaly useful
Mental Agility: Max 5 points, reduces the casting cost of instant spells by up to 10%
Since many of your spells are instants this can provide a nice reduction in the time you spend out of mana.
Improved Mana Burn: Max 2 points, reduces the casting time of your mana burn spell by up to a second.
By lowering the casting time by 1/3 you are also upping your damage output of that spell by 50%, since you can now cast it 3 times in the same amount of time it took for 2 castings.
Very useful if you are using the spell all the time.
Situational
Tier 5
Mental Strength: Max 5 points, increases your total Intellect by up to 15%.
Can add a significant amount of mana at high levels.
Scales extremely well with gear, as 15% is a huge amount for any talent.
Divine Spirit: 1 point skill, increases the targets spirit for 30 minutes.
80 spirit (at max level) adds considerably to a players mana regeneration rate and is very useful for long fights or grinding.
Improved Divine Spirit: Max 2 points, your Divine Spirit and Prayer of Spirit spells also increase the target's spell damage by an amount equal to 100% of the Spirit granted.
This is a situational talent, if you are casting these spells on casters regularly then it can be a benefit, especially in raids.