Extension methods in .Net

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.

exp:

namespace ExtensionMethods
{
  public static class MyExtensions
  {
      public static int WordCount(this String str)
      {
          return str.Split(new char[] { ' ', '.', '?' }, StringSplitOptions.RemoveEmptyEntries).Length;
      }
  } 
}

Ref link:
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http://msdn.microsoft.com/hi-in/library/bb383977(en-us).aspx