Our extension methods are used all over this book, but under this chapter we will describe them all. We will also show some samples so that you can create your own extensions. The
Custom Extensions chapter cover that topic in depth.
Extension methods as described on MSDN:
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type. For client code written in C# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type
If you are new to extension methods read more on MSDN