A simple case:

public class Foo
{
     public /*virtual*/ bool DoSomething() { return false; }
}

public class Bar : Foo
{
     public /*override or new*/ bool DoSomething() { return true; }
}

Call the code like this:

Foo a = new Bar();
a.DoSomething();

NOTE: The important thing is that our object is actually a Bar, but we are storing it in a variable of type Foo (this is similar to casting it)

Explain:

New keyword is for Hiding. - means you are hiding your method at runtime. Output will be based base class method.

Override for overriding. - means you are invoking your derived class method with the reference of base class. Output will be based on derived class method.

Check out the  following link for more discussion:

http://*.com/questions/159978/c-sharp-keyword-usage-virtualoverride-vs-new

C# Keyword usage virtual + override VS new