ASP stands for Active Server Pages , I.E. creates Active (or interactive) Web Pages live on the Server before they are passed to your browser.
PHP is very similar to ASP but is an Open Source product. Both operate in the same way.
Because the code runs 'on the server' and not on your 'local' machine, it is a very powerful and fast programming system that puts no strain on your machine, and because it runs on the web server, it does not matter what type of machine or browser you view the web pages on.
When you call an ASP page from your browser, the Server gets the page and runs the code before passing it back to the users browser as a pure HTML page. As HTML is the browser's native language, there is no need to download plug-ins or applets.
ASP is a Microsoft programming language which can access and store data in a data base located on the web server, eg. this allows the storage of product information in a central data base, the web pages then read this information and produce HTML pages with the correct product information and prices displayed on them. The data base can be updated by the web owner from his home or office, and all information on the web site will be updated IMMEDIATLY.