normal beams spread all over the road and up as well, where as rREAL fog lights have a cut off and are made to light up only the lower portion of the road to aid in foggy conditions.
if you put on your high beams in fog all you'll see is more fog since high beams light up air higher up and that is where most of the fog is (like F1 said)
Sadly, nowdays foglights are being confused with driving/ aux lights which are there to aid your headlamps and have a much wider spread beam, and therefore are better to light up the road in normal conditions.