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.
What is GPX?
GPX is the XML that GPS devices and fitness apps speak. A file is usually one recorded track - a long list of points with latitude, longitude and often elevation and a timestamp - plus any waypoints you marked along the way.