glFusion uses a callback handler function style plugin api to allow plugins to interact with and modify core code functionality. All callback handler functions for each plugin is located in its functions.inc file. The below functions are what you need to write your own plugins, including any extra parameters that you need to pass in, as well as a code example to get you started.
Based largely off of information originally located at http://www.geeklog.net
glFusion version of this documentation: