What is KMZ?
KMZ is simply a zipped KML, and it is what Google Earth actually exports - the zip lets one file carry its images and icons alongside the geometry. The geometry converts; the artwork inside does not, and the result says how many pieces were left behind.
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.