• Google Earth is in a Geographic coordinate system with the wgs84 datum. (EPSG: 4326)

• Google Maps is in a projected coordinate system that is based on the wgs84 datum. (EPSG 3857)

• The data in Open Street Map database is stored in a gcs with units decimal degrees & datum of wgs84. (EPSG: 4326)

• The Open Street Map tiles and the WMS webservice, are in the projected coordinate system that is based on the wgs84 datum. (EPSG 3857)

So if you are making a web map, which uses the tiles from Google Maps or tiles from the Open Street Map webservice, they will be in Sperical Mercator (EPSG 3857 or srid: 900913) and hence your map has to have the same projection.

I'll like to expand the point raised by mkennedy

All of this further confused by that fact that often even though the map is in Web Mercator(EPSG: 3857), the actual coordinates used are in lat-long (EPSG: 4326). This convention is used in many places, such as:

• In Most Mapping API,s You can give the coordinates in Lat-long, and the API automatically transforms it to the appropriate Web Mercator coordinates.
• While Making a KML, you will always give the coordinates in geographic Lat-long, even though it might be showed on top of a web Mercator map.