Traits are a feature of your Sim which confers special behaviors, social interactions and abilities to your Sim. You can buy them with Lifetime Points and will each fill up one trait slot.
Each trait features several levels, with each subsequent level costing more Lifetime Points than the one before.
To buy a trait, choose the traits button underneath the picture of your avatar. This brings up the traits box, where you can buy / upgrade traits or change your Personality.
You can have up to 6 traits equipped at a time. You start with one open slot. You gain the ability to buy a new slot at levels 3, 7, 12, 20, and 40. All the trait slots have a cost, but it varies on your Sim's current trait level.
List of traits
There are 13 types of traits. Click the image to read more about each type.