For some time now, Google Maps has supported KML. This means that much of the content you create in Google Earth (such as placemarks, overlays, etc.) can be viewed in Google Maps as well.
However, KML can move from Maps to Earth as well. If you are viewing KML-based content in Google Maps, you can export the KML out as a file that you can view in Google Earth. Just look for the KML icon and link in the top right corner of the Google Maps page.
Example: here is a mountain bike ride I did late last year. I tracked my ride using a GPS device, saved the GPS data to KML using Google Earth, then posted the KML on a web server. Notice the KML link in the Google Maps web page.
Another example: Any Google Maps content (such as this depiction of Kauai) created using My Maps can also be exported as a KML file and brought into Google Earth.