Programming Microsoft Dynamics CRM 4.0: Plug-ins
Microsoft Dynamics CRM 4.0 offers a powerful means of extensibility through plug-ins, which you can register directly into the event execution pipeline. You can register and deploy plug-ins by using existing tools or by implementing your own deployment tools using an API. Because Microsoft Dynamics CRM implements plug-ins with no dependencies on specific class implementations, they are a good target for automated unit tests. The number of messages that can be registered for is significantly larger than any previous version of Microsoft Dynamics CRM and allows developers to extend CRM much further than was possible in the past.