Traits modify different aspects of gameplay, such as primary statistics, derived statistics and skills. Traits have positive and negative results, unlike perks in earlier Fallout games. In games where traits exist, one can choose up to two traits when designing a character (or none at all). Choosing the Mutate! perk lets the player change one of their traits. Note that if you chose no trait at character creation, you cannot fill the empty slot with this perk.
In Fallout Tactics, where characters of different races can be created, traits also have different racial requirements.
List of traits
Old World Blues traits