KML is a format for geospatial data. It's based on XML in the same way that GeoJSON is based on JSON: a KML file is a particular structure and kind of XML file. KML was created by Keyhole, a company Google acquired and turned into Google Earth, and is now maintained by Google and the OGC.
KML has some unique features, mostly by virtue of being designed with Google Earth in mind. It can store styles alongside geometry. It can store "flight paths" for automatically panning and sweeping through maps. KML files can also contain NetworkLink objects, which let them refer to other KML files that could be on the internet or local.
KML can contain styles for features - a line in KML can be "red" or "blue". This is a relatively unique feature in geospatial data - see the format feature matrix for official style support. KML styles are transformed into simplestyle when they're imported into Placemark and you can view them by changing your map's visualization mode to simplestyle.
Have feedback or a question that isn't answered here? Contact us by email.