[c#] What is an .axd file?

What kind of purpose do .axd files serve?

I know that it is used in the ASP.Net AJAX Toolkit and its controls. I'd like to know more about it.

I tried Googling for it, but could not find getting basic information.

An AXD file is a file used by ASP.NET applications for handling embedded resource requests. It contains instructions for retrieving embedded resources, such as images, JavaScript (.JS) files, and.CSS files. AXD files are used for injecting resources into the client-side webpage and access them on the server in a standard way.

Those are not files (they don't exist on disk) - they are just names under which some HTTP handlers are registered. Take a look at the web.config in .NET Framework's directory (e.g. C:\Windows\Microsoft.NET\Framework\v4.0.30319\Config\web.config):

<configuration>
  <system.web>
    <httpHandlers>
      <add path="eurl.axd" verb="*" type="System.Web.HttpNotFoundHandler" validate="True" />
      <add path="trace.axd" verb="*" type="System.Web.Handlers.TraceHandler" validate="True" />
      <add path="WebResource.axd" verb="GET" type="System.Web.Handlers.AssemblyResourceLoader" validate="True" />
      <add verb="*" path="*_AppService.axd" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="False" />
      <add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="False"/>
      <add path="*.axd" verb="*" type="System.Web.HttpNotFoundHandler" validate="True" />
    </httpHandlers>
  </system.web>
<configuration>

You can register your own handlers with a whatever.axd name in your application's web.config. While you can bind your handlers to whatever names you like, .axd has the upside of working on IIS6 out of the box by default (IIS6 passes requests for *.axd to the ASP.NET runtime by default). Using an arbitrary path for the handler, like Document.pdf (or really anything except ASP.NET-specific extensions), requires more configuration work. In IIS7 in integrated pipeline mode this is no longer a problem, as all requests are processed by the ASP.NET stack.