You can use "Cookies" to store certain information for a given period of time on your visitors' computers for later reference. For example, if one of your visitors view a special page at your site you can set a cookie on that user's browser indicating this fact, so that when he/she comes back you'd know that he/she has been to a given page at your site.

Learn all about how to "bake cookies" by reading Netscape Cookies and Privacy FAQ and HTTP Cookies Specification. Then, if you like to implement cookies using JavaScript go on to JavaScript Cookies. If you prefer to use Perl instead, try Perl5 CGI Library.