Active Server Pages or ASP, a language I actually started off programming in, is a technology that enables you to make dynamic and interactive web pages.
ASP uses server-side scripting to dynamically produce web pages that are not affected by the type of browser the web site visitor is using.
ASP pages have the extension .asp instead of .htm, when a page with the extension .asp is requested by a browser the web server knows to interpret any ASP contained within the web page before sending the HTML produced to the browser. This way all the ASP is run on the web server and no ASP will ever be passed to the web browser.
Any web pages containing ASP cannot be run by just simply opening the page in a web browser. The page must be requested through a web server that supports ASP, this is why ASP stands for Active Server Pages, no server, no active pages.
As ASP was first introduced by Microsoft on it's web server, Internet Information Services (IIS), that runs on Windows 2000/XP Pro/NT4, it is this web server that ASP pages usually run best on.