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.
In Tcl, ncgi is a module in tcllib that deals with CGI. Another useful package is Don Libes's cgi.tcl extension.
[List other useful references - perhaps like Apache's mod_dtcl, etc.]
I've been told that BOOK CGI Developer's Resource - Web Programming in Tcl and Perl is quite a good reference on CGI and Tcl.
[I should make a point of explaining clearly that there are several distinct approaches to CGI coding with Tcl, including:
...]
[ Fast CGI (punctuation?) is also likely to be of interest. ]