Update: Sign up is no longer required for Google Earth Pro. Click the link to download Earth Pro, then sign in using your email address and the License Key GEPFREE.
There are several differences between the free version of Google Earth and Google Earth Pro. The free version of Google Earth lets you print screen resolution images, whereas Google Earth Pro offers premium high resolution photos. The free version of Google Earth requires you to manually geo-locate geographic information system (GIS) images, but Google Earth Pro helps you automatically find them. And the free version of Google Earth only allows you to import image files that are up to a max texture size, but Google Earth Pro offers Super Image Overlays that are more than the max texture size.
Google Earth Pro uses the same imagery as the free version of Google Earth. However, Google Earth Pro offers additional tools designed for business users like the ability to create animation movies and an option to set up measuring areas of polygons or circles. Google Earth Pro also lets you map multiple points at once and lets you access demographic, graphic and traffic data layers.
It says free trial in the link but it seems to work fine in the program.