The ArcGIS Server Web ADF for the Microsoft .NET Framework (.NET Web ADF) is an AJAX-enabled framework for building powerful, easy to use applications that display, analyze, and edit GIS data stored on a central server. It includes a set of Web controls, frameworks, and APIs that can be used to build an application from the ground up or to customize an application you created using the ArcGIS Server Manager Web application.
If you are just getting started, you can learn about using the .NET Web ADF to create a map or task.
You can also take a look at the samples. Many samples are available that show some aspect of the .NET Web ADF.
The Library Reference is also available if you need more information about each object and its properties, methods, and events.
