The talent system is designed to allow you to customize your character by giving you new abilities or enhances your old ones. Every time you level up you receive a talent point which can then be used to buy a new ability in one of the three talent trees. Arcane, Fire and Frost. Each tree is broken into tiers stacked on top of each other. In order to put points into tier two of a tree you must have first spent at least five points in that tree. In order to put points in tier three you must spend at least ten points. Also, some talents also require that before you spend point in that talent you have to spend points in another, usually in a lower tier.
All of this is important because you will only have a maximum of sixty one points to spend and must choose carefully which talents you want to invest in. The most powerful ones are lowest down in the trees because of their requirements and prerequisites. But this also means you will be mostly limited to that one trees specialty and it will severally influence and limit your play style. Since some talents are not very worthwhile or even worthless, talent builds were developed to make sure that your sixty one points are spent as efficiently as possible.

Arcane Subtlety
Description:
Reduces your target's resistance to all your spells by an amount and reduces the threat of your arcane spells.
Ranks:
Rank 1- 5 spell penetration, 20% reduced threat.
Rank2- 10 spell penetration, 40% reduced threat.
Usefulness:

Commentary:
How effective this talent is for you largely depends on what you will be doing. It's always good to have a little spell penetration to increases your DPS. In a raid or group environment though your targets will usually have a curse reducing their resistances and there is no such thing as negative spell penetration. Since arcane spells are very mana intensive though it is always good to avoid a resist. And this applies to all resists so fire and frost builds may want to consider it as well. The threat reduction is much larger than you get for the other schools of magic. But you will have to be focused on pure arcane damage to make full use of it.

Arcane Focus
Description:
Reduces the chance opponents can resist your arcane spells by a percentage
Ranks:
Rank 1- reduces resist chance by 2%
Rank 2- reduces resist chance by 4%
Rank 3- reduces resist chance by 6%
Rank 4- Reduces resist chance by 8%
Rank 5- reduces resist chance by 10%
Usefulness:

Commentary:
This can be a very useful talent if you use arcane spells a lot. Since they are usually so mana intensive it is always good to avoid resists. It effects other arcane spells like polymorph and counterspell too which are very important in PVP and small group play. In a raid environment though you will only find it useful if you use lots of arcane damage. If you do though, it will let you focus on stats other than spell hit.

Improved Arcane Missiles
Description:
Gives you a chance to avoid losing casting time when casting arcane missiles.
Ranks:
Rank 1- 20% chance to avoid interruption
Rank 2- 40% chance to avoid interruption
Rank 3- 60% chance to avoid interruption
Rank 4- 80% chance to avoid interruption
Rank 5- 100% chance to avoid interruption
Usefulness:

Commentary:
Arcane missiles can deal some very high burst damage if you are geared correctly, but if the target is attacking you you will lose some of your spell and make it less effective. If you are grouping with any kind of tank you won't need this, but if you are playing solo it is helpful. It is very strong in PVP because you will have a spell in your arsenal that is completely immune to spell pushback, letting you use it against melee types when you can't escape.

Wand Specialization
Description:
Increases the damage you deal with wands.
Ranks:
Rank 1- Increases damage by 13%
Rank 2- Increases damage by 26%
Usefulness:

Commentary:

Wand damage is a joke compared to your other spells and you will only use one if you have no other options or need to return mana. Save your points and put them into one of the other first tier talents.

Magic Absorption
Description:
Increases all of your magical resistances and restores a percentage of your mana when you fully resist and enemy spell.
Ranks:
Rank 1- Increases resistances by 2 and restores 1% of your mana.
Rank 2- Increases resistances by 4 and restores 2% of your mana.
Rank 3- Increases resistances by 6 and restores 3% of your mana.
Rank 4- Increases resistances by 8 and restores 4% of your mana.
Rank 5- Increases resistances by 10 and restores 5% of your mana.
Usefulness:

Commentary:

Magical resistances have been severally downplayed since the burning crusade expansion but they can still be useful in certain raid boss fights. The mana restore ability is also nice if you are in one of these fights and have enough resistances. The five talent cost can be somewhat prohibitive so you may want to make this a filler talent when trying to get deeper down the tree and not fill it out all the way.

Arcane Concentration
Description:
Gives you a chance of entering a clear casting state when a damaging spell hits a target. The clearcasting state reduces the mana cost of your next spell by 100%
Ranks:
Rank 1- 2% chance
Rank 2- 4% chance
Rank 3- 6% chance
Rank 4- 8% chance
Rank 5- 10% chance
Usefulness:

Commentary:
This is excellent for increasing your mana efficiency. When it procs you can use your high mana spells like arcane missiles more often. It is also useful if you depend on arcane blast for your damage since the mano cost grows higher as your use it.

Magical Attunement
Description:
Increases the effect of you amplify magic and dampen magic spells.
Ranks:
Rank 1- Increases effect by 25%
Rank 2- Increases effect by 50%
Usefulness:

Commentary:
Amplify and dampen magic are very very situational spells and this is really a waste of two talent points. The only reason you might put points in this is to increase the healing of amplify magic on boss fights that have no magic damage.

Arcane Impact
Description:
Increases the critical strike chance of your arcane explosion and arcane blast spells by a percent.
Ranks:
Rank 1- 2% increased critical strike chance.
Rank 2- 4% increased critical strike chance.
Rank 3- 6% increased critical strike chance.
Usefulness:

Commentary:
This is only useful if you are mainly doing arcane damage. Arcane explosion is a very useful move in PVP so the extra crit is worth while. The extra crit on arcane blast is also good if you use it to deal most of your damage. If you don't use these spells often or are only putting points into arcane as support for your fire and frost spells skip this.

Arcane Fortitude
Description:
Increases your armor by an amount equal to 100% of your intellect
Ranks:
One rank
Usefulness:

Commentary:
This may seem like a really stupid talent, but it does have some limited application. By itself it is weak, but if you combine it with prismatic cloak, frost warding and ice armor you'll reduce the melee damage you take by a fair amount. Sometimes it will be just enough to save you in a PVP situation if your enemy is undergeared.

Improved Mana Shield
Description:
Decreases the mana lost per point of damage taken when mana shield is active by a percentage.
Ranks:
Rank 1- Reduces mana lost by 10%
Rank 2- Reduces mana lost by 20%
Usefulness:

Commentary:
The only time you will be using mana shield enough to make this worthwhile is in PVP. In that case it is nice, but you should still rely on ice black and ice barrier first.