The addition of traits was one of the more subtle changes between 3.5 D&D and Pathfinder and one which I initially overlooked. From a pure rules standpoint, they are not that impressive. Each trait is theoretically equivalent to half of a feat. Characters get to pick two traits during character creation normally, although there are a few ways to get another trait if needed.
What makes traits so cool is that they are a rare blend of setting and rules. Traits can add flavor to a character while giving them some stat benefit. Traits can give guidance/suggestions on how a character fits into the world. Often when I'm running a game off of a Paizo adventure path, I will require one of the two traits be chosen from the companion player's guide. Picking an adventure path trait will instantly help a character it into the world. Traits can also give additional class skills, which is a boon when trying to build flexible characters. There is one limit to traits: traits come in multiple categories, and you can have a max of one trait from a given category.
Here are a couple of examples:
Dangerously Curious
You have always been intrigued by magic, possibly because you were the child of a magician or priest. You often snuck into your parent’s laboratory or shrine to tinker with spell components and magic devices, and frequently caused quite a bit of damage and headaches for your parent as a result. You gain a +1 bonus on Use Magic Device checks, and Use Magic Device is always a class skill for you.
Andoren Freedom Fighter
You’ve dedicated your life to fighting against oppression, tyranny, and slavery. You gain a +1 trait bonus on attack and damage rolls against slavers or any creature holding someone against their will.
See more here.
In conclusion, traits can add a good deal of flavor and aid in differentiating characters beyond just stats. I hope I've given you enough incentive to consider traits and important part of your character. If you have any questions, please leave a reply and I'll be happy to answer them.
I keep wanting to use two complementary bits of terminology that may not be common knowledge, so I thought I would explain them.
Crunch: The rules/stats portion of a game. Str, Dex and Con stats are all crunch. Crunch is defining a character as a level 4 rogue, or level 10 paladin. Feats, traits, base attack bonus, saving throws and class features are all crunch.
Fluff: The setting and flavor portion of the system. Fluff is not about the numbers, it is the description of the world and how the characters fit in the world. Character fluff is not a level 4 rogue, but a mid-level member of the Red Mantis Assasin's guild. Character fluff is not a 10th level paladin, but a high ranking Paladin of Iomedae who hails from the great temple in Taldor.
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