virtual function

Hi,
I am learning Virtual function in c++. I read Virtual function is member function that is used to achieve run time polymorphism. It is known that polymorphism means one name but multiple forms.But how run time polymorphism(i.e;one name multiple forms) is achieved through virtual function?

Please help me in understanding the concept.

Also,RUNTIME TIME POLYMORPHISM and METHOD OVERRIDING are synonyms?