Stuck fermentation?

I've heard the term "stuck fermentation" on this site a few times and was wondering what exactly is it? What are the factors that contribute to it and how can it be fixed?

Stuck fermentation is when your yeast quit before it is fully fermented out. If your starting gravity is, for example 1.052 and all the yeast drop out after a few days and you then take a hydro reading and its now at 1.020 when you figure it should go to like 1.012 This can happen if the temp drops too much.... the yeast think its time to flocculate and save themselves. It may help to raise the temp a little and/or try to rouse the yeast back into solution.

It can also occur if your original gravity is rather high (1.08+) and the yeast you use can't handle the high alcohol level. In that case, wait for the normal fermentation to finish then pitch some champagne yeast which can handle up to 20+ % alcohol but contribute very few phenols/aromatics.