The answer to this question is kind of long but here is everything in a nutshell.
First, you do NOT need an application like VS or VWD to create ASP.NET pages it just makes life alot easier. (You could, for example, code all of your asp.net pages in Notepad ;] )
Second, Gonzalo is 100% correct, ASP.NET is just a technology (just as ASP was) and there are many languages out there that you use to program against that technology; c# and vb
.net being the most popular.
Lastly, without any custom configuration to IIS, your ASP.NET pages will end in .aspx (You can of course map any file extension to the .net api's for execution)
And i think this definition taken from Professional ASP.NET 2.0 Server Control and Component Development
by Shahram Khosravi
sums it up:
ASP.NET is a Framework that processes requests for Web resources.
In other words, ASP.NET is a request processing architecture or framework. This description of the Framework prompts you to think, “If ASP.NET is a request processing architecture or framework, every component of the framework must exist for one reason and one reason only; that is, to contribute one way or another to the process of handling requests for Web resources.”
Chapter 1 - Overview
Read this if you want to know how to get a correct reply for your question:
Technical Editor for:
Professional Search Engine Optimization with ASP.NET
Professional IIS 7 and ASP.NET Integrated Programming