For the British army at least, the difference is that, in terms of stature, the Heavies were bigger men on bigger horses. In an age when it was extremely unusual to be over 5'10", this was a minimum for the heavy cavalry regiments. They must have appeared to be giants, a bit like seeing a basketball team today. Their horses were also bigger (taller). In terms of difference in usage - light cavalry were trained in piquet duty (guarding outposts)and scouting, occasionally being involved in skirmishes with their enemy counterparts. British heavy cavalry spent much of the Napoleonic wars at home in Britain, it was only at Waterloo that they finally got to put their training into practice - namely, providing a massive attack in a large-scale battle. Incidentally, both light and heavy British cavalry had a poor reputation when it came to obeying the trumpet calls and reforming in a disciplined manner (an essential basic skill for all cavalry). Instead, perhaps because of their fox-hunting, aristocrat, dandy officers, they preferred to charge on and on for glory, which generally meant they were wiped out by their much better-disciplined French counterparts.