The Common Gateway Interface is a simple process based method of calling scripts to generate dynamic web content. However, there is significant overhead in creating a Process for every client request.
FastCGI (aka FCGI) was implemented to use TCP sockets to provide CGI like data to a long-lived process. This opens up other high performant possibilities such as in-memory caching, database connection pooling etc... PHP is a popular web scripting language with a sturdy FCGI implementation.
FCGI is a bit of a long-winded and over-complicated spec which scared a lot of people away for a long time, but it is coming back into fashion at the moment.
SCGI (Simple? CGI) does essentially the same thing as FCGI with a much simplified socket protocol.
The CGI Servlet provides bindings for Process based CGI, or Socket based FastCGI. SCGI coming soon.
For now see the javadocs for the CGIServlet and CGIFactory for a description of various initParams.
Also see the Jetty-PHP example.
If you are hoping to use the CGI Servlet to process URIs like /context/script.cgi/path/info then the PathInfoFilter will be of interest.