KML files are plain ASCII text files. The standard format that Google Earth will save a placemark is .KMZ which is a zipped version of a .KML file. If you have Google Earth^® installed on your computer, it should open as you double click on a .KML or .KMZ file. Sometimes KML files from the server will open as text file in the browser. If so, the server does not send the correct MIME type for these files

You can make an Apache server send the correct MIME type
by adding the following lines to either the
httpd.conf file or the .htaccess file:

AddType application/vnd.google-earth.kml+xml kml
AddType application/vnd.google-earth.kmz kmz

This and similar useful infos from beginner to advanced programmer
you find in the KML tutorial, the KML reference and on the Google Earth Community pages

If you place a .KML file or a .KMZ file on your server, a link to this file should open a window like this
on the client computer. If the client browser shows the file as text or opens an unzipper, the server does not send the correct MIME type.

simple kml - file

minimum kml file for placemarks

multiple placemarks in one file

defining buildings in a kml file

overlay maps on Googles Earth

watch and play route in Google Earth

fix if Google Earth does not connect  to server