White Walkers according to those that live in the 7 Houses are just a myth. Those that defend the Wall and are unlucky enough to serve the Nights Watch however, know very well that White Walkers are real. Little is known about the white walkers, as they tend to not attack the Wall or come into contact with others very much. The only ones that have actually seen a white walker in person are those that participate in near daily scouting missions beyond the Wall. The few contacts with white walkers that have came from these scouting always end badly, usually with one or more members of the scouting party getting severely injured in the process. At this point in time there is nothing to fear about White Walkers, but it is up the the Nights Watch to defend the Wall and prevent the White Walkers from invading again.
They are considered extremely dangerous if you come into contact with them, and it is advised to contact the local crows if you somehow run into one. When there is one... there are dozens. They are faster, stronger, and know the terrain better then you. You'll be dead before you know it.