ASP.NET is a web application framework
developed and marketed by Microsoft,
that programmers can use to build
dynamic web sites, web applications
and web services. It was first
released in January 2002 with version
1.0 of the .NET Framework, and is the successor to Microsoft's Active Server
Pages (ASP) technology. ASP.NET is
built on the Common Language Runtime
(CLR), allowing programmers to write
ASP.NET code using any supported .NET
language.
ASP.NET (Wikipedia)
That's on the second result searching on Google so I'm guessing (half-expecting) that you don't understand what that means either.
Webpage development started with simple static HTML pages. That meant the client asked for a page by means of an URL and the server sent the page back to him/her exactly as it has been designed. Sometime after that several technologies emerged in order to provide a more "dynamic" or personalized experience.
Several "server side languages" were developed (PHP, Perl, ASP...) which allowed the server to process the Web page before sending it back to the client. This way when a client requested a webpage the server could interpret the request, process it (for example connecting to a database and fetching some results) and send it back modifying the contents and making them "dynamic". The fact that the process took place on the server stands for the name of "server side".
So the original ASP (predecessor of the ASP.NET) was a server side language that was focused on serving web pages. In such way it supported several shortcuts such as the possibility to intercalate HTML and ASP source into the file which was on that time much popular due to PHP implementation. It was also (as most of these languages) a dynamic language and it was interpreted.
ASP.NET is an evolution of that original ASP with some improvements. First it does truly (try to) separate the presentation (HTML) from the code (.cs) which may be implemented by using Visual Basic or C# syntax. It also incorporate some sort of compilation to the final ASP pages, encapsulating them into assemblies and thus improving performance. Finally it has access to the full .NET framework which supports a wide number of helper classes.
So, summing up, it is a programming language located on the server and designed to make webpages.