[This documentation is preliminary and is subject to change.]
In Windows SharePoint Services 3.0, you could host a Microsoft Silverlight application in a Web Part. Microsoft SharePoint Foundation 2010 goes beyond by providing a built-in, extensible, Silverlight Web Part specifically designed to host Silverlight applications. Closely related to the new Web Part is the Fluid Application Model (FAM) that enables secure, cross-domain, integration between external applications and SharePoint Foundation deployments.
Silverlight Web Part
Adding a Silverlight application to your SharePoint Foundation solution need not require any SharePoint Foundation development at all in the simplest case: Users install your Silverlight application on the same domain as their SharePoint Foundation Web application and add the hosting Silverlight Web Part through the UI. All they need supply is the URL of the application. If the Silverlight application accesses SharePoint Foundation data and is hosted on a server outside the domain of the Web application, you create External Application XML that users, in turn, use to register the hosting Silverlight Web Part. A built-in Silverlight Tool Part is also part of SharePoint Foundation 2010. For more information, see SilverlightWebPart, SilverlightToolPart, How to: Create External Application XML Markup, and the other topics under the Web Parts that Host External Applications Such As Silverlight node of this SDK.
Fluid Application Model
It can be a great advantage to SharePoint Foundation users to be able to host applications that are in a different domain from the SharePoint Foundation Web application, because many such applications can be hosted on an application server and made available to all Web applications in the farm. SharePoint Foundation 2010 introduces the Fluid Application Model to make this scenario possible in a secure way. The FAM enables administrators to control the permissions of the external applications without unduly restricting the ability of users to add Web Parts hosting these applications to Web Part pages. The application logs on to the SharePoint Foundation Web application as a distinct type of user known as an application principal. The application’s permissions are the intersection of the permissions that the administrator has granted this special user and the permissions of the real user who opened the Web page containing the Web Part that hosts the application. For more information about development with the FAM, see Overview of Integrating External Applications with Web Parts and the other topics under the Web Parts that Host External Applications Such As Silverlight node of this SDK.