The world is full of innumerable entities. Without the ability to generalize -- forced to approach the world as if every entity were entirely unique and different -- you would waste all of your time grasping fundamentals over and over again. Life would be impossible.
A concept is a mental abstraction which allows generalization and the extension of knowledge from some known objects to others unknown. It integrates two or more particulars into a common mental unit. For example, the concept "book" subsumes all particular books. It does so based on the essential characteristics of multiple pieces of paper or pages combined into a bound stack.
A concept is formed by taking a number of similar entities and deciding what makes them similar in an important way. The differences and the unessential similarities are not important and are abstracted away from the newly created mental entity. Each concept serves a particular purpose and is created to allow higher-level thinking. People do not waste their time forming arbitrary concepts.
While concepts integrate particulars, concepts can also act as particulars. In this way, it is possible to form higher levels of abstraction, combining concepts into more complex concepts, and furthering one's understanding and knowledge by increasing the amount that can be integrated. Higher level concepts can also allow more complicated combinations that are not possible by trying to integrate lower level particulars. The concept wife is not possible without the concept marriage, the concept relationship, and so on down a long tree of complex concepts.
Although a concept is built from particular entities, it is not tied to those specific entities. If those entities were changed or destroyed, the concept would still be intact, but would no longer include those particulars. The concept combines any entities with those particular characteristics. It encompasses any entity with those particular characteristics, past, present, or future.
There are two essential tools to complete the concept formation. The first is a definition. This is the method of specifying the essential characteristics of the concept, or what is the basis of the integration. It also specifies the method of differentiation, which distinguishes it from everything not encompassed by the concept.
The second tool is a word. A word is a cognitive trigger for the concept. It is the method by which the concept is stored away and referenced later. Without such a trigger, the act of integrating each concept would have to be redone every time it was used. It would be difficult to form higher level concepts, and the level of integration of one's knowledge would be severely limited in scope.