.NET is a framework for web applications

NET is a framework, or software platform, from Microsoft which you use to develop and execute application programs. Microsoft’s .NET is an open platform to combine the various threads which have influenced software development in the last few years and which ended up developing in different directions. Customers should once again be able to use a single technology. .NET runs on the Windows operating system: it is automatically shipped with Windows customer operating systems such as Windows 7 or Windows 8.1, and is pre-installed. In Windows Server operating systems the .NET framework is included but not pre-installed.

.NET creates a run-time environment for programs

The .NET framework expands Windows with a variety of components, the most important of which is the runtime environment, also known as common language runtime. All programs which were previously developed with the .NET framework run in the runtime. The developed program will not be able to start without this run-time environment. Further components of the framework are object-oriented class libraries, programming interfaces and utilities. These components are important for the developer to create programs using .NET and let you create extensive functions for graphical user interfaces, web interfaces, database access and much more.

You can find more information on .NET on Wikipedia: https://en.wikipedia.org/wiki/.NET_Framework

or on the official Microsoft .NET website: https://msdn.microsoft.com/en-us/vstudio/aa496123