It's implemented as a Java servlet using Kawa Scheme for run-time page compilation. (It's a template engine like PHP or JSP.) Pages are recompiled when modified; it's very easy to develop with.
Read all about it at brl.sourceforge.net
[Isn't CLiki supposed to list only Common Lisp resources?]
Yes, but I think it said "Lisp" when I first put this in here. I'm removing the link from "web resources" but won't erase everything on the page in case someone links to this.
Daniel Lowe took the syntax idea (without the web server framework) over to a Common Lisp library he named Stencl.