From MSDN Web Site i took this:
The following table lists the choices you have for writing and deploying plug-ins and custom workflow activities.
Tool Description and usage More information
Plug-in and Custom Workflow Activity classes
Plug-in Registration Tool
The plug-in and custom workflow activity classes allow you to create event handlers to perform custom business logic that you can integrate with Microsoft Dynamics CRM to modify or augment the standard behavior of the platform.
By using these classes directly, your code will not contain any helper code provided in the Developer Toolkit.
If you write plug-ins and custom workflow activities from scratch, you must use the Plug-in Registration Tool to register them. This tool provides a graphical user interface and supports registering plug-ins and custom workflow activities with Microsoft Dynamics CRM.
Use this method if you:
Understand how to use the plug-in and custom activity classes.
Don’t want extra library code auto-generated and placed in your code files.
Don’t mind using an external tool and the web application to register and package custom code assemblies.