Addons is the collective term for all PHPFusion's Infusions, Themes, Panels, Plugins, Widgets, BBCodes, User Fields, Graphics or any other PHPFusion based additions such as Core modifications or similar additions.
You can find Addons in the Marketplace
Any Addon that utilizes infusion.php to modify the Core database is called an Infusion.
Themes control the layout and the graphical design of your site's front-end. Admin Themes controls the style and the graphical design of your Administration area.
Themes go to root/themes/THEMENAME
Admin Themes go to root/themes/admin_themes/ADMIN_THEMENAME
Any Addon that creates either a center or side panel is defined as a Panel.
Panels go to root/infusions/PANELNAME_PANEL
Any Addon that are designed by the Widgets API will be defined as a Widget. Widgets are used in Custom Pages.
Widgets go to root/widgets/WIDGET
All other Addons including BBCodes, User Fields, Graphics and Core modifications are simply defined as others.
BBCodes go to root/includes/bbcodes/BBCODENAME_bbcode_include ( two files + locale ).
User fields go to root/includes/user_fields/user_FIELDNAME_include ( two files + locale ).