Usually only domesticated cats meow, whereas wild, feral cats do not. Domesticated cats also don't tend to meow to one another. This suggests that cats meow as a way of communicating with humans.
A domesticated cat meows to get the attention of a human, perhaps because the cat wants food, because it wants to be let outside, or maybe simply because it would like some attention.
In a scientific paper published in 2009, Karen McComb, a professor of animal cognition and behavior at the University of Sussex, suggested that when a cat meows it is taking advantage of a human's natural, nurturing response to the cries of a baby. In other words, cats may have learned to adapt their meows to mimic the cries of a baby, so as to elicit more effectively the attention they desire from their human owners.