Skunking is described in this article as UV radiation caused breakdown of "hop derived molecules, called isohumulones", which then bond with sulfur, giving you a skunk-like smell.
As other articles have said over and over, keep your beer cool and dark. You should have a safe, cool, dark storage place for your beer to ferment (if you're like me without temperature controlled fermentation), condition, and age if you so choose. If you fear several of your bottles have been exposed to too much light, you can then have a taste test with one from your "cellar" (laundry room closet for me).
The article also mentions the misinformed opinion that Heineken, and other German or dutch Pilsner style beers are "skunked", when in truth the flavors common to them are intended, having nothing to do with spoiling or light exposure.
After being challenged in the comments to compare a bottle and can of Heineken, I have to concede, I and the article were wrong. I did purchase from a specialty beer and wine store, but their display cases weren't much different than a gas station. I poured both into a glass, which should have normalized the experience (refer to the article). I immediately noticed the smell difference, and took them to my wife, who was easily able to pick out the skunk smell without knowing anything about my experiment. The taste is remarkably similar, considering how much smell can affect taste. The smell is not a question, bottle skunk confirmed. I love my Grolsch bottles, but I'll be extra careful to keep them in the dark from now on.