Late Harvest wines should really be called Later Harvest wines, as they simply are wines that have been left on the vine longer then the would be usually. As they are left longer on the vine they develop more flavour development – flavour-producing compounds are found in the skin so the riper the skins can get the more flavour develops. This basic concept explains why Chardonnay, for example, has tropical and stone-fruit flavours in warm regions like California, but have citrus and apple flavours in cool regions like Burgundy in France.
Along with flavour development that increases the longer the grapes stay on the vine there is also an increase sugar development as well – though this is minor. Usually Late Harvest wines will just be left on the vine longer to get an increase in flavours then not fermented until dry, resulting in a sweet wine with more flavour.
If you are familiar with German and Austria wines you will have noticed the words Kabinett and Spatlese. Both of these wines are made as Late Harvest wines, though Kabinett grapes don’t spend as long as Spatlese grapes do.