The wraiths are not, as some claim, a projection of an inner fear. They are visible, tangible and dangerous on top of that. The priests teach that people who die suddenly, leaving this vale of tears with important tasks left unfinished, become such ghosts. So wraiths have their own aims. Sometimes they are unaware of them, but more often they aim to achieve them, not caring for the living.
The wraiths are always connected to the sites of their demise. They may protect their former homes, or they may seek vengeance. One may even find small groups of the wandering damned in crypts and catacombs or on battlefields. It is not true that they only appear at night. I can confirm, however, that one cannot communicate with them, and that they have no fear of man or monster.
Wraiths hover just above the ground, so they move very swiftly. One should strike at them strongly and make ample use of Signs. As immaterial beings, they can appear and disappear at will. If the witcher swings his sword too slowly, giving the ghosts time to react, he might find himself striking nothing but air and then sense a quick riposte on his back.
Wraiths do not bleed, and common poisons cannot harm them. They are, however, susceptible to fire and of course to silver. The best means of fighting a wraith is the so called Specter Oil, which should be used to coat a silver blade.