What is a KML fileKML (Keyhole Markup Language) is an XML based file format used to display geographic data in an Earth browser such as Google Earth, Google Maps, and Google Maps for mobile.
With KML, you can display pretty much everything on a map. You can either email it to friends or you can upload it to Google Maps (Google My Maps feature) and share it with everyone.
You can find out more about KML in Google’s developer site or the KML Tutorial also from Google.