The cPAddons system allows a script to be packaged up so that it can be installed on a user's website, upgraded, uninstalled and otherwise managed from cPanel and WebHost Manager. This manual describes technically how to package a script as a cPAddon and how to maintain that script.
Any script that you wish to install on multiple websites could potentially be a cPAddons script. A guestbook, content management system, blog engine, etc could all be cPAddons scripts. The benefit to having cPAddons scripts is that these scripts can be installed by cPanel users from their cPanel interface after the cPAddon script is installed on the server.