Quite simply, .NET is Microsoft's platform for XML Web services. XML Web services allow applications to communicate and share data over the Internet, regardless of operating system or programming language.
The Microsoft .NET platform includes a comprehensive family of products, built on XML and Internet industry standards, that provide for each aspect of developing, managing, using, and experiencing XML Web services. XML Web services will become part of the Microsoft applications, tools, and servers you already use today—and will be built into new products to meet all of your business needs.
More specifically, there are five areas where Microsoft is building the .NET platform today, namely: Tools, Servers, XML Web Services, Clients, and .NET Experiences.
Web sites vs. XML Web services
Web sites are about presenting information to a user: they are the communication vehicle for servers to talk to users. XML Web services offer a direct means for applications to interact with other applications. Applications hosted internally, as well as on remote systems, can communicate via the Internet by using XML and SOAP messages.