Passive skills are inert skills gained by players, which are not cast as skills. For example, an increase in an attribute such as MP, or the ability to use a specific weapon. Once the skillbook is used, the player automatically continues to have the passive skill. Each class has passive skills, although some apply to more than one class.
To check the skills that have been learned, go to the Skills Window (K) and click on the <Passive> Tab. (See image).