In some of the questions on this site, I've seen the word 'skunking' thrown around. What does it mean? I'm assuming it's a process that causes beer to go bad, and if so, what causes this process?
Skunking is a stage in a beer's life in which it goes rancid. It may smell funny, almost like a skunk, and taste bad. As John pointed out, it is generally caused by excess exposure to light. Beers packaged in clear (Corona and Land Shark for instance) or green bottles are particularly susceptible to this. It is best to avoid these kinds of beers to avoid the risk of skunked beer; choose beer bottled in brown bottles or cans, which are less susceptible to light exposure.
Some of the mocelues in the beer are broken down by ultraviolet light and bind to sulfur atoms. This causes Skunking. This is why some bottle six packs have a tall sleeve and beer keeps longer in a darker colored bottle. A common misperception is warming and cooling over and over again causes Skunking.