What are Virtual Functions? How to implement virtual functions in C++ ?

refer
complete reference for c++
by herbert schildt..

object-oriented programming (OOP), a virtual function or virtual method is a function whose behavior, by virtue of being declared "virtual," is determined by the definition of a function with the same signature furthest in the inheritance lineage of the instantiated object on which it is called. This concept is a very important part of the polymorphism portion of object-oriented programming (OOP).

For Detailed study read any manuals or refer any book.As this is a core feture of OOPS,you can find everywhere