Traits in Divinity: Original Sin Enhanced Edition track the personality of a character to determine certain statistical bonuses that are given to you.
These are accessible in your character menu. They are displayed in pairs of 2, and a sliding meter shows towards which side is your character is progressing. The bonuses are applied when one side of the trait has higher points than the other.
For example, "Pragmatic" increases your Reason score by 1 point, whereas "Romantic" (on the opposite side of that scale) increases your Luck score by 1 point. You can never receive a bonus from Pragmatic and Romantic at the same time. If you (for example) make a dozen Pragmatic choices and never choose a Romantic choice, you will only have a +1 in Reason gained from being Pragmatic a dozen times (and not +12 as you might expect). In order to ever be considered Romantic (and earn a +1 in Luck), you'd have to make 13 Romantic choices (and no further Pragmatic choices). 
|Left Bonus||Left||Right||Right Bonus|
|+1 Willpower||Independent||Obedient||+1 Willpower when an ally with Leadership is in sight (+2 Willpower if ally has Leadership 5 or higher)|
|+3% Critical Chance||Compassionate||Heartless||+20% chance to hit when backstabbing|
|+2 Reputation||Altruistic||Egotistical||+1 Bartering|
|+1 Leadership||Righteous||Renegade||+1 Pickpocketing|
|Immune to Charmed||Blunt||Considerate||+1 Charisma|
|+1 Initiative||Bold||Cautious||+1 Sneaking|
|+1 Crafting||Pragmatic||Romantic||+1 Lucky Charm|
|Immune to Cursed||Forgiving||Vindictive||+20% chance to hit on attacks of opportunity|
|Immune to Fear||Spiritual||Materialistic||+1 Loremaster|
- ^ Larian Studios Forum Post - Post #480704: http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=480704