This article defines ASP.NET and links through to some official documentation and tutorials.
ASP.Net is the server-side library for C#Bot which does the following:
- manages the request/response pipeline
- handles controller actions
- manages user authentication
Overall, ASP.Net allows the code to be executed by an internet server.
ASP.NET extends the .NET platform with tools and libraries specifically for building web apps. (See What is ASP.NET?)