Barking might just be a reflex for agitated dogs. It might be a side-effect of domestication — i.e., when you select for less-aggressive animals you get ones that tend to bark. Or, it really might have meaning, both for other dogs and for humans. At Scientific American, Brian Hare and Vanessa Woods describe some of the research on dog communication, including studies that show both humans and dogs can tell the difference between barks associated with, say, food, and barks associated with the presence of a threatening stranger.