Some extension installers offer installer hooks for the installing application, such as the component installer. This system replaces the older 'com_install' and 'com_uninstall' methods that used to exist instead using an object to contain this information.
The class name of the item should be the element name followed by "InstallerScript". So for "com_alpha" the class name would be "com_alphaInstallerScript".
Install and uninstall are analogous to to the old "com_install" and "com_uninstall" functions. Update is a new function handled for update situations and "discover_install" is a special version of install that is used when the extension is 'discovered'. Finally preflight and postflight accepts a param display what sort of operation we're doing (one of the above options: install, uninstall, update, discover_install) and is triggered before different operations take place.