One of the initial disadvantages of the web was that it was very static. Each and every person saw the exact same thing if they visited the same URL. This has changed dramatically over the course of this decade, however, thanks in large part to server-side scripting languages and technologies, such as ASP, Perl, CGI, and others.
However, customization is limited to programs on the server. Small tidbits of information, known as cookies, can be stored on the client (the browser) to provide a customized web experience. Cookies can be used to store when the last time someone visited your site, what pages they visited, what ads they clicked on, and other useful, profiling information.