From A Wiki of Ice and Fire
Ravens are a species of large, black-feathered birds native to Westeros. Similar to common ravens of Earth, only stronger with better homing instinct. Ravens are used to carry messages between castles and cities. While doves and pigeons can also be trained to carry messages, ravens are a stronger flyers, larger, bolder, far more clever, and better able to defend themselves against hawks.
The maester of each castle usually tends to its own flock of ravens. Maesters raise a breed of large of white ravens at the Citadel that are said to be more intelligent and are sent out to mark the change of seasons. Ravens sometimes imitate human speech like parrots. Ravens can speak the True Tongue, the language of the children of the forest, a tongue no human can.
The logistics of how ravens deliver messages is that most ravens are trained to fly to a specific castle. Some few can be taught to fly between two castles. Such birds are greatly prized. Once in a very great while, a raven can learn the names of three or four or even five castles, and fly to each upon command. Birds as clever as that come along only once in a hundred years
In the godswood at Raventree Hall in the Blackwood Vale there is a dead ancient weirwood of colossal size. Every evening at dusk hundreds of ravens come and roost on the tree all night, as they have for thousands of years.