The world, unlike the universe, consists of individual things and of all other kinds of entity. It consists of individual things and of properties. The world consists of individuals and of a number of categories of abstract things. This chapter shows that there are altogether seven categories, namely, individuals, properties, relations, structures, sets, quantifiers, and facts. Structures differ from other kinds of complex thing by the fact that they can be isomorphic to each other. Structures are things in relations. Without relations, there are no structures. One should think that this fact alone calls attention to the importance and categorial stature of relations. Facts, in distinction to some individuals, all properties, all relations, and all numbers are complex. One thing is clear from the beginning: the world is a complex entity. The world may be either a set of facts, a structure of facts, or a fact consisting of facts.