Each placemark (or set of placemarks) is described in a KML file. Within the description of each placemark in the KML file, you can use HTML much in the same way you would if you were creating a web page. Background colors, tables, etc.
According to the KML tutorial:
The most important thing you need to know about authoring your own HTML in placemarks is the use of the tag. If you want to write standard HTML inside aRead more about using descriptive HTML. Or you can see how others have done this by looking at the KML used in other placemarks (see my posting on 10/19). And of course, you can change the icon used by the placemark in the 3D viewer (learn more here).
tag, you really should put it inside a CDATA element. If you don't, the angle brackets will need to be written as entity references to prevent the Google Earth from parsing the HTML incorrectly. (This is a standard feature of XML applications and not unique to Google Earth.)