Microsoft CRM 4.0 Architecture

The platform is the heart of the Microsoft Dynamics CRM system. When you use the Microsoft Dynamics CRM SDK, you are building on top of this system. The Microsoft Dynamics CRM platform supports smaller deployments and can scale for application service provider models also. The security mode protects the platform.

The main platform components are as follows:

• Microsoft SQL Server database
• Web services
• System services (workflow, metadata, and integration)
• A query processor that supports the entity model
• Secured ad hoc queries that use an XML fetch statement to protect the physical database
• Plug-ins for business logic extensibility
• Reporting services.

When you develop an application that uses the Microsoft Dynamics CRM server, you use Web services to communicate with the underlying platform layer.
The server platform is responsible for creating domain-specific objects. In Microsoft Dynamics CRM, these objects include contact, lead, opportunity, account, business unit, and more. The goal of the platform is to implement the service-specific rules by manipulating and combining the underlying domain objects.
The platform does not impose business-specific logic. This layer imposes only generic domain constraints. It contains the building blocks for an application, but by itself is nothing more than a collection of related objects. However, the interaction between those objects within the domain can be assumed to implement more extensible logic such as the quote-to-order-to-invoice processing and pricing logic.
The server platform also controls access to objects through security, controls access to the database, and raises events for workflow processes and custom business logic implementations. The platform layer provides for both incoming and outgoing e-mail processing through Microsoft Exchange.

http://msdn.microsoft.com/en-us/library/bb928229.aspx