.NET is a programming framework created by Microsoft that developers can use to create applications more easily. .NET provides applications in an orderly way so that you are able to access databases, web services, and other communication tools.
It helps you make better applications in a fraction of the time. Less time has to be taken to make trivialities and more time to build a functional application that meets the requirements. Things that would normally take a lot of efforts like reading/writing on disk, getting data from a database, etc, are so trivial that it doesn’t even compare to the competitor.
The .NET Framework is a huge library that is shared with all the other applications that use it. It’s so big that including it with an application would make no sense. For that reason, the .NET Framework has to be installed on your computer before you use the app.