When we talk about inheritance in object-oriented programming languages, which is a concept that was invented in 1967 for Simula, we are usually talking about a way to form new classes and classes are instances of which are called objects and involve using classes that have already been defined. 
Derived classes  are intended to help reuse existing code with little or no modification and  are the new classes that  take over (or inherit) attributes and behavior of the pre-existing classes, usually referred to as base classes (or ancestor classes). 
Categorization in computer languages is a powerful way number of processing information and inheritance provides the support for representation by categorization. Furthermore,  it is fundamental for helping humans learn by means of generalization in what is known about specific entities is applied to a wider group given a belongs relation can be established and cognitive processing which involves less information being acquired to be stored about each specific entity, but in actual fact only its particularities.
An instance of a "fruit" is a generalization of "apple", "orange", "mango" and many others. Inheritance can also sometimes be referred to as generalization, because is-a relationships represent a hierarchy amongst classes of objects. It can be considered that fruit is an abstraction of apple, orange, etc. Conversely, since apples are fruit, they may naturally inherit all the properties common to all fruit, such as being a fleshy container for the seed of a plant.
 Modules with sufficiently similarities in interfaces would be able to share a lot of code and therefore reducing the complexity of the program. This can be known as one of the advantages of inheritance. Therefore inheritance can be known to have a further view, a dual, which describes many parts of code that are under control of shared control code, named as polymorphism. 
On the other hand, inheritance is normally accomplished either by replacing one or more methods exposed by ancestor, or by adding new methods to those exposed by an ancestor. A well known term used for this replacing act is called overriding.