Active Server Pages, a.k.a. ASP, is a Microsoft® solution aimed at Web-based application development. It allows the combination of HTML, scripts, and ActiveX® components.
Having delivered tools that work with ASP, I'd like to say that they do work for delivering custom content. Basically when someone hits an ASP page, you run script on the server that dynamically generates the page. Embedded within the HTML on the server, the script tags <% and %> mark sections of script code that execute on the server, providing dynamic content to the page.
Mostly used for database query and display of the resulting recordsets. You can use Microsoft® Visual Basic® Scripting Edition (VBScript) or Microsoft® JScript (essentially JavaScript). Other languages such as Python and Perl may be used via plug-ins. Sun Microsystems copied ASP more or less wholesale to create JSP.
No matter what you think of MS, and I know most of you hate them, many corporate customers use their stuff. Being able to do basic ASP is valuable, especially since JSP is so alike, and more palatable for Java purists.
Nonetheless, we approach ASP projects with the following quote:
ASP? Very dangerous. You go first!