KML or Keyhole Markup Language file is based on an XML language which stores  and models geographic information.

What it contains:

A KML file may consist of both vector and  raster data  with different geometry types.

What application can be used:

Google Earth, QGIS, ArcGIS, Global Mapper, 

The Open Geospatial Consortium provides the standard specifications and definition for a KML file and can be accessed through this link.

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