Difference between a KML and a KMZ file
A KML file is a "flat" file or text based file composed of Tags similar to XML or HTML. A KMZ file is basically a ZIP archive that contains a KML file. This was done so that your any software such as Google Earth would automatically recognize it as a file and open it. If they had left it as a .ZIP file, then it wouldn't automatically open the file. You can actually create a KMZ file yourself. Create your KML file then use WinZip to compress the file. After this is done, rename the file extension to .KMZ. After you do this, try double clicking on the .KMZ file you just created. This should automatically open in Google Earth.