MapServer TEMPLATE
It helps me to understand a concept if I write about it. Here we go.
"Templates" is the simplest way to create an interactive mapserver site. (You can create non-interactive ones without templates but what's the fun of that?)
Without even using a template, I (the masterful Web GIS progreammer) can create a .map file and an HTML template and put the appropriate URL onto a Web page, for example,
http://mapserver.wildsong.biz/cgi-bin/mapserv?map=../mapserver/html/hello.map&mode=map&layer=credits
This invokes the CGI version of the mapserv executable that lives on my server and tells it to load up a map file, display the results by sending the image back to your browser ("mode=map"), and to display what's in the layer called credits ("layer=credits").
To get it to use my template file, I can change mode to browse and add a template specifier: