What is KML?

Google Earth's format, and the reason it looks good - unlike GPX or GeoJSON it carries styling, so lines have colours and placemarks have icons. Still XML underneath, and still the thing to convert to when someone wants to see a route draped over terrain.

What is GeoJSON?

The format web maps speak - points, lines and polygons as plain JSON, with arbitrary properties hung off each feature. Leaflet, Mapbox and every mapping library read it natively, which makes it the usual destination for map data that came from somewhere else.