mod_lisp is an extension module for the Apache HTTP Server. It enables Apache to interface with application servers written in Lisp, making it possible to dynamically generate web pages and provide web applications with Lisp programs.
Unlike other mod_* projects, such as mod_perl and mod_python, mod_lisp does not embed a Lisp interpreter in Apache. In concept it is closer to FastCGI, handling requests by serializing them into a textual representation and sending that over a socket to a persistent Lisp program, and then sending the Lisp system's response to a client. The client may be a web browser, but is more likely to be a reverse proxy server. It is this server that interacts with the user's browser.