CGI is an abbreviation for Common Gateway Interface. http://www.w3c.org/ is one body which proposes and approves standards for internet applications - see [quote reference] for CGI related standards.
Benny Riefenstahl: See the Apache FAQ: "Where can I find the 'CGI specification'?" [L1 ].
In Tcl, ncgi is a module in tcllib that deals with CGI. Another useful package is Don Libes's cgi.tcl extension. ncgi, along with html-htmlparse, and later javascript, essentially factors cgi.tcl, while introducing namespace amenities.
[List other useful references - perhaps like Apache's mod_dtcl, etc.]
[I should make a point of explaining clearly that there are several distinct approaches to CGI coding with Tcl, including:
FastCGI is also likely to be of interest.
To the extent that CGI is used in the casual sense of "dynamically generated web pages, or forms handling, or Web application", there is also AOLServer, with its built-in TCL interpreter and database hooks, and nstcl, which is a TCL-only extension which has AOLServer's API to http queries and database handling, as well as a templating library.
As is Apache Tcl and Rivet.
Tclhttpd supports dynamically generated content, and also optionally implements the vanilla CGI API as one of its Tclhttpd Domains.