This type is a delegate and is pretty cool. It can contain a method with one parameter and no return value. You can read a bit more here: http://msdn.microsoft.com/en-us/library/018hxwa8.aspx
This type is used Transient Fault Tolerance Framework in some of the ExecuteAction methods. For instance, the following signature allows you to call ExecuteAction and pass in 3 anonymous methods:
public abstract void ExecuteAction(Action<AsyncCallback> action, Action<IAsyncResult> callback, Action<Exception> faultHandler);
You can see an example of calling this method in this article: http://msdn.microsoft.com/en-us/library/hh545245(v=VS.103).aspx