Mastery Trees allow you to customize your summoner to whichever way you want it to be. When summoner reaches level 30, you have the choice to construct full Mastery Trees. When you build these, the most important thing to keep in mind before you use your talent points is which summoner spells you plan on using. Each summoner spell has an upgrade that can be gained through Masteries.

If you're just starting the game or if you're just a low level, there are a few talents you should begin with. All of them are in the Utility tree. 3/3 Good Hands reduces the time you spend dead by 10%. Awareness, which you obtain on the second tier in the Utility tree, increases your exp gain by 5%. As a new/low level player this can help you gain levels quicker. You are allowed to respec during the load-screen by minimizing your screen and going to your PVP.net client.

Here's the stat boosts that you get if you spec into all of the stat boosts in each of the different trees.

Offense Tree:

Critical Strike Chance: +2%
Ability Power per Level: +0.60
Cooldown Reduction: -3%
Attack Speed: 4%
Magic Penetration: +15%
Armor Penetration: -6
Damage to Minions: +4
Attack Damage: +3, +5%
Critical Strike Damage: +10%
Magic Damage: +5%

Summoner spell upgrades

Exhaust: -25 armor, -25 magic resist, +.5 second duration
Smite: +5 gold, -5 second cool-down
Promote: Siegerider +20 armor, taunts towers, -15 second cool-down
Rally: +20% magic damage, +5 second duration

Defensive Tree:

Magic Resistance: +6
Armor: +6
Health Regeneration: +0.30% of Max Mana
Dodge: +2%
Physical Damage Reduction: +2
Minion Damage Reduction: +2
Damage Reduction: +5%
Health: +60

Summoner spell upgrades

Heal: +75 heal, -10 second cool-down
Revive: +400 health for 90 seconds, -20 second cool-down
Boost: -30 second cool-down, +2 second duration
Fortify: +6 damage to minions when off cool-down

Utility Tree:

Death Timer Reduction: -10%
Health Regeneration: +4%
Mana Regeneration: +4%, +1/sec
Experience Gain: +5%
Mana: +5%
Gold: +0.10/sec
Neutral Buff Duration: +30%
Movement Speed: +3%
Cool-down Reduction: -6%
Summoner Cool-down Reduction: -15%

Summoner spell upgrades

Teleport: -.5 second cast, -5 second cool-down
Ghost: +8% movement speed, +1.5 second duration
Flash: -15 second cool-down
Clairvoyance: 30 second invisibility vision, +30 second duration

Offense tree is mainly for Physical DPS (Damage per second). Defensive tree is for tanks, and the Utility tree is for Magical DPS. If you spot any mistakes in the guide please don't be shy to tell me! Thanks.