Keyhole Markup Language (KML) is an XML based format for describing geospatial features and symbols for viewing in Google Earth.
KML can be created manually, or by a program that usually converts GIS data to KML.
These KML samplescan also help you get started creating.
There are many programs available to help you convert existing GIS data (such as shapefiles) into KML. Many free scripts for ESRI's ArcMap can be downloaded off of ESRI's scripts page.
Arc2Earth provides a professional package for creating KML in ArcMap.
Shape2Earth provides professional conversion without the need for ArcMap or any other licensed software.