The word “skunked” is often used to describe beer that has been spoiled. However, skunked refers to a specific type of spoilage that is caused by beer being exposed to light. Skunked and light-struck are synonymous terms.
Skunking has nothing to do with temperature, oxidation, or bacterial infection, though all of these can spoil beer as well.
The hops used in beer are sensitive to light in the 450 to 520 nanometer range, which is in the blue-green part of the visible spectrum. This means that virtually any light source, from the sun to fluorescent lights, is bad for beer.
The light causes riboflavin to react with and break down isohumulones in the hops which creates molecules which are chemically similar to the mercaptans that give skunk musk its less than appealing smell.
The reason that most beer is bottled in brown bottles is the brown glass is reasonably good at blocking the blue-green light. Green bottles offer very little protection and clear bottles offer no protection at all. Now you know why beers brands which use clear bottles are often served with lemon or lime; it helps cover the bad taste caused by the fluorescent lights in the beer coolers at the store.
Some beers are said to be made with hop extracts which have had the isohumulones removed, making them impervious to skunking, but I have found little evidence to support the claim. If you can point me to some verifiable evidence, please feel free to school me in the comments.