Attributes are skill and class masteries. Passive attributes improve a skill's effects or grant a class with unique effects and features. Active attributes modify a skill's behavior and can be toggled ON or OFF. Attributes possess their own levels, you can level them up by training them. To do that, you need to talk to your class NPC. Training an attribute requires time and money.
List of Attributes
For a complete list, consult TOSBase's database.