This service uses Shuttle Radar Topography Mission, version 2, data from NASA. It has a resolution of ~90m (latitude dependent). Most of the world's landmass (99%) is covered between -60 and +60 degrees as standard. For high latitude regions such as Scandanavia and Iceland, extra tiles have been added courtesy of Jonathan de Ferranti at viewfinderpanoramas.org.
To enhance the terrain data there are two clutter layer options available. You can either add a layer of random clutter at a set height or upload your own defined clutter as a KML layer. Minimum width for each item of clutter is 70m.
Public domain SRTM data is imperfect and contains a few radar blackspots. A radar blackspot is where a high feature such as a mountain obscured the ground behind it from view of the Space shuttle's synthetic radar. For this reason, agencies who use it for life and death decision making such as Air Forces use their own refined elevation data. Google Earth uses refined elevation data (but still isn't 100% accurate) so a quick way of verifying if there are any terrain discrepancies is by checking it against Google Earth.