Many scripts make tools that auto-load into their menus/submenus etc every time Sketchup starts.
Often they'll consist of one .rb file that goes into the Plugins folder, sometimes they might also use some subfolder files, like png's that are used for the tool's toolbar etc.
Some plugins - usually the more complex toolsets - are written as 'Extensions'.
These consist of a 'loader' .rb file and a subfolder containing all of the main .rb/.rbs files and other files like html, png etc used to make webdialogs, toolbars etc.
The 'loader' defines the tool[set] as an 'Extension' - this is recognized as Sketchup starts and depending on how the extension is 'set' it loads or doesn't load the main code.
An Extension can be activated/deactivated from the Preferences > Extensions dialog.
This is useful for tools that might be infrequently used, because this will improve startup time because unneeded code isn't then auto-loaded unless the Extension is activated.
You can activate an Extension tool at any time and its menus/toolbars etc are added to the current Sketchup session.
If you don't deactivate an Extension it will continue to auto-load at startup every time until you deactivate it again...
A deactivated Extension only ceases to be loaded after a restart.
As well as letting you manage which tools load [without you having to 'disable' individual files in the Plugins folder, by moving or re-suffixing them etc] the Extension method has other uses... the new .rbz archive format tools [shipped with recent v9 updates] allows the .rbz file's contents of Extensions and their subfolders to be automatically loaded and activated at the click of a button...
Many of us are in the process of making our more complex tools into Extensions, and using the .rbz format to allow easy installation/updating; however, it can be time consuming to recode existing tools, so progress is slow...
