Ravens were large black birds of Middle-earth, sometimes associated with the ill news of death, and were known for their harsh voices. The Ravens of Erebor often brought secret news to the people of Thror, the King under the Mountain, and in return they were rewarded with bright objects to hide in their dwellings dotted around Dale and at Ravenhill. Ravens could evidently live to great ages, with 153 years being the oldest recorded.
The ravens that resided at Ravenhill were capable of speech and some could even speak Westron. These specific ravens were in alliance with the Dwarves of Erebor. Carc, a notable raven chief, had been one of these along with his son Roäc who continued his father's leadership around TA 2941 - when Thorin and Company reclaimed Lonely Mountain (Erebor). Roäc was described in The Hobbit as being a most decrepit bird. He was getting blind, he could hardly fly, and the top of his head was bald. He was an aged raven of great size.
It has been suggested that J.R.R Tolkien's use of ravens, especially Roäc, is reminiscent of Odin's two messenger-ravens Huginn and Muminn of Norse Mythology, from which Tolkien drew many of his ideas.