A file with the .KML extension is a Keyhole Markup Language file. KML files use XML to express geographic annotation and visualization by storing locations, image overlays, video links, and modeling information such as lines, shapes, 3D images, and points.
Learn Google Earth: Importing KML, KMZ, and GPS Data
Several geospatial software programs use KML files because the goal is to put the data into a format that other programs and Web services can easily use. This included Keyhole Earth Viewer from Keyhole, Inc. before Google acquired the company in 2004 and began using the format with Google Earth.
Google Earth was the first program that could view and edit KML files, and it's still one of the most popular ways to open them online. With the web page open, use the Projects menu item (the pin/map icon) to load a KML file from your computer or Google Drive account.
ArcGIS, Merkaartor, Blender (with the Google Earth Importer plugin), Global Mapper, and Marble can also open KML files.
