As per official asp.net website, Asp.Net Web Forms are "When users request a page, it is compiled and executed on the server by the framework, and then the framework generates the HTML markup that the browser can render. An ASP.NET Web Forms page presents information to the user in any browser or client device". In layman language, Asp.Net Web Forms are another way of creating web pages and do server side programming using C#.Net. You can make web pages, create a complete web application.
It is very easy to build applications in Asp.Net Web Forms. It's designer is very competitive and provides a very clean way to design the Web UI. You can easily drag componets or you can switch to code view to write your own HTML. It provides various options to cater different type of developers. The Visual Studio in itself is a powerful too to do a lightening fast development, so Asp.Net web forms inherit that quality from Visual Studio
Short answer is any web application with not 100 users at a time, that includes some, but not all:
LD have been actively involved in developing various
web application projects built in Asp.net Web Forms.
In earlier era of technology wave, Asp.net web forms proved to be a very big hit,
competing with PHP as its main competitor.
Us being an avid Microsoft follower, we chose Asp.net as our development platform.
There had been many successful implementation ever since.
We adopted Asp.Net strongly and suggested our clients to use Asp.net
Web Forms for a quick delivery and easy solution.
We have delivered around 80 projects involving asp.net. Couple of our projects you can look at