Jeszcze lepszy przykład; Vob to szef, a wszystko inne to jego pracownicy. 
This is called "inheritance". Each class that "inherits" from zCVob (i.e. is a subclass of), "inherits" a) the data and b) the functionality. So because a zCVob has a member called "visual", all subclasses will have it as well (obviously mobs need a visual, so do NPCs, Items, ...). Also all Vobs have some sort of "functionality", e.g. you can probably "hide" a vob (it would turn off the visual). Inheritance implies that you can also hide NPCs, Mobs, Items, ...
Each subclass will become more specific. While a zCVob is just any visual object, a oCMobInter is more than that: It's a visual object that is also movable (hence "Mob") and interactable (hence "Inter").