What is a Sketchup plugin?
There are many Sketchup plugins out there to help you modeling. Some plugins export the model to other formats and some help you to draw things and speed up some processes. Most plugins are just a basic text file but some come with icon images and some even with dll:s and other files.
How to install
Just place the plugin file(s) in the plugin folder. Usually something like 'C:\Program files (x86)\Google\Google Sketchup 8\Plugins\'.