What is KML?
KML is Google Earth's format, and by extension the one most mapping tools will import without argument. It describes placemarks, paths and polygons, and it can carry styling - colours, icons, line widths - alongside the geometry.
What is GeoJSON?
GeoJSON is what geographic data looks like when it stops pretending not to be JSON. It is the lingua franca of web mapping - Leaflet, Mapbox, D3 and most spatial databases read it directly - and its one famous trap is coordinate order: longitude first, then latitude.