Overriding a method is to change the behavior of the method for the derived class.

Overriding is done using inheritance and virtual functions. 

Example

To explain the concept of overriding I have created two classes Poly

and abc(inherits poly).

    public class poly
    {
    //these two functions, sum(), are overloaded.
        public int sum(int x, int y)
        {
            return (x + y);
        }
        public virtual int sum(int x, int y, int z)
        {
            return (x + y + z);
        }
    }
 
    public class abc : poly
    {
   //overriding function sum(), of base class poly
        public override int sum(int x, int y, int z)
        {
            return (x + y + z + 10);
        }
    }