The image or the symbol of a raven can mean different things for different people.
1. Believers in Wicca
People who practice Wicca (modern witches) believe that the Raven is a mystical bird who can travel between the world of the living and the dead. Therefore, the knowledge the bird holds is special given they possess knowledge of both worlds.
2. Native Americans
Native Americans believe that the Raven possess the knowledge one needs to enter the place where the Great Spirit lives. Therefore, the raven symbolizes a great change.
Pagans believed the raven to symbolize a healing and protection. The raven is a symbol which shows change is coming.
The Druids believed that the raven possessed the knowledge of what was to come (omens). The Druids believed that by studying the bird they could predict what was to come.
5. Ravens have also been used to depict the morbid and Gothic. Given they have been used in many horror and tales of suspense, the raven has been depicted in many texts such as Poe's "The Raven." Historically know to be extremely intelligent, the bird has been used as a messenger bring knowledge of death, sorrow, and what is to come.