To caw is to make a loud cry or call, like the sound a crow makes. There are few things spookier than hearing fifty crows caw in your yard at night.
Many large birds caw as a way of communicating with each other, but crows and ravens are best known for making this particular cry. You can call the actual sound a caw as well. The word has been around since the 16th century, when it arose as an imitation of the noise itself.