There are two kinds of inheritance. They are Generalization
and specialization. Generalization is bottom to up function
and specialization is top to down function. 
Example: We have one class. this class is called Employee.
and then employee class includes the following class. There
are Programmer,Staff and Manager. Because employee class
have ID,Name, and salary. If u want to call programmer class
to employee that is called generalization. and if u want to
call employee class to staff that is called specialization.