Cats communicate with other cats primarily by using body language. But they have figured out that humans communicate mostly by vocalizing, and have adapted. Studies have shown that house cats vocalize a lot more with us than they do with one another. What clever kitties!
The specific meanings of various felines meows and chirps have not been very well studied, but a group of scientists at the University of Sussex in England published a study this summer (in the journal Current Biology) that looked into the meaning of certain purrs.
They found that sometimes cats just purr, but when they need something, they add a little crying meow underneath the purr.
They recorded the purrs of cats who were asking for something and cats who were not, and when they played them back, even people who had never lived with cats could hear the difference and knew the crying purr meant the cat was asking for something. The scientists also found that the crying sound was a lot like the sound of a baby crying, and suggested that cats may have figured out that we humans (like lots of other mammals) tend to respond to baby sounds.