This is a handy option for displaying Google Maps in Google Earth. This could be any of your own maps created in Google Maps, a route or whatever. One nice feature of GE's Network-Links is that if you or any other contributor of a Google map modifies it (in Google Maps), it will be refreshed also in Google Earth.To do so, just grab the link from Google Maps. Like this one for a route:
Then go to GE and choose "Add" from main menu and "Network-Link" from the submenu. Then paste the above link and append "&output=kml" to it. That's it!