OK, so we don't know exactly what they're saying, though I suspect that at least some of the time they are yelling, "Give me a peanut, human!" But this fantastic video from ornithologist Kevin McGowan helps you recognize the difference between crow and raven calls, as well as what they could mean.
Anyone who has spent just a little time around crows knows that these birds are always talking to each other. When I put peanuts out on my back porch, the first crow to arrive will spend at least a minute yelling at the top of its lungs — then I'll hear some distant replies, from down the black, and suddenly there are five crows in the tree instead of one. McGowan explains that the typical "caw caw" cry seems to be a way for the birds to claim territory, but also to alert their family group to their location. Crows can sometimes use a call that sounds like clicking or chopping, and that seems to signal location as well.
Ravens have a much louder and more deep-throated cry, and they probably use it in much the same way crows do. They're calling out to their family, possibly letting them know where food is. Or, as McGowan notes, they might just be chuckling to themselves.