Levels

Basics
In order to level up, you must acquire soul shards by defeating enemies, and then you must use them at a statue. When you level up, you get three [insert correct term] level up points which you can allocate to the four character attributes. Also, your Life (HP) and Attack Power (Damage) will increase. Every five levels, you will get another trinket slot from leveling up as well.

Vigor
Increases your maximum life and makes you less likely to be stunned

Savagery
Increases weapon damage and the chance to stun enemies

Mastery
Increases attack speed and the chance to deal critical damage

Focus
Increases energy gained from soul shards and items. Gives additional trinket slots.

Recommended Builds
Generally, it is agreed that the most important attributes to upgrade are mastery and savagery. A recommended end game build is 12 Vigor / 25 Savagery / 26 Mastery / 10 Focus