Google Earth has been available as a browser plugin since May of 2008. A plug-in is a software package (in this case: Google Earth) that adds specific capabilities to a larger software application (your browser: Firefox, Internet Explorer 8, Safari). HTML is displayed in your browser: all images in this window, the text you are reading now, the navigation bar above, and the plugin with control buttons below. When you select a tour, the code will load a KML file (Keyhole Markup Language) into the plugin. KML defines coordinates in a reference system (eg., Earth, moon, sky) and displays them from the locations as placemarks. The tours you will see have timed courses of flight, viewing angles, and bubbles programmed by HTML with images, text and links to resources. You will need to download the Google Earth Plugin to run the tours from your machine.