If you have a look at the Building Objects and Advanced OO techniques you will see that there is mention of polymorphism. It will help you to read this. There are also samples with these chapters that demonstrate it.
I can also recommend that you read Dr GUI on MSDN.
Dr GUI is great for beginners, as there are many basic ideas and terms expressed in the column.
Here is one of the columns that explains topics such as Inheritance, Polymorphism and interfaces.
I hope this helps.