.NET Framework 3.5 defines multiple Application Programming Interfaces (API) that developers use as the programming foundation for their applications.

The .NET Framework includes individual components that together provide features for enhanced applications. The .NET Framework 3.5 APIs are grouped into feature sets that include:

• Windows Presentation Foundation
• Allows applications to draw to the screen using standard methods defined in the Windows Presentation Foundation. The Windows Presentation Foundation unifies the look and feel of the operating system for developers.
• Windows Communication Foundation
• Allows applications to communicate with each other using the standard methods defined in the Windows Communication Foundation. The Windows Communication Foundation enables applications to send messages to each other
• This API is used in Windows 7 for communication between standardized Web services, peer-to-peer sharing features, Really Simple Syndication (RSS) support, and new core networking services. It connects users and their applications to the services they need, when they need them, anywhere over a network.
• Windows Workflow Foundation
• Allows developers to build applications that follow a logical sequence of events. The sequence of events, or workflow, a user takes to complete a task is guided by the business logic based on what they need to accomplish. The Windows Workflow Foundation is a programming model that allows developers to quickly build workflow-enabled applications. As multiple applications share in the processing of data, the Windows Workflow Foundation can help answer the question of "What happens next?"
• Windows CardSpace
• Protects a user's digital identities. Windows CardSpace is a part of the .NET Framework 3.5 model that protects a user's digital identities. Windows CardSpace allows for applications to keep track of a user's security credentials (user ID and password) for one or more security systems. Each set of credentials becomes one information card assigned to the user. Windows CardSpace is specifi- cally hardened to prevent identity theft and spoofing (pretending to be someone else).

Read more about the .NET Framework at: http://msdn2.microsoft.com/en-us/netframework/aa663309.aspx