It's hard to talk about, work with, or even think about dogs without broaching the topic of barking. Even the absence of barking is frequently noteworthy.
As a matter of fact barking can be a very big issue. The sort of issue can lead to dogs being rehomed or...worse. Coppinger and Feinstein ("Hark! hark! The dogs do bark ... and bark and bark"; Smithsonian 1991 - can't find a link.) once recorded a dog barking for seven hours straight.
Why do dogs bark? From an evolutionary context, that's a good question. While wolves and coyotes are capable of barking, it is very rare. Belyaev's famous foxes famously do, much like dogs. (Silver foxes normally do not bark very often.)
I can think of four main "types" for barks off the top of my head, and since that is exactly where this blog entry is coming from, those are the four I will list.