Wraith is the Scottish word for 'ghost' or 'spirit'. Wraiths are usually undead beings which are incredibly fast and love to kill. They can be in any form, though they rarely take that of a human. In some areas they can fly and are hideous, simply skin and bone thin and can sometimes look like Furies is vary rare cases. To be completely honest, there is little information on Wraits, so you are allowed to make up anything about them, though in other cases they are regarded as aliens.
Some places you may have heard of Wraiths may be:
Dungeons and Dragons (a type of undead)
Primal (video game)
League of Legends (mmorpg online game)
Comic books (X-men, Spider Man, Image comics superhero)
Wraith: The Oblivion (rpg)
Stargate (a hostile alien race)
In the third Shadow Dance series by author David Dalglish, the Wraith is a humanoid figure that likes to kill people and is small, yet incredibly fast.
In Stargate, the Wraith are a vampiric hive-based species that harvest the 'life-force' of other humanoid beings for nourishment through suckers on their right hand palm. Countless worlds in the Pegasus galaxy live in constant fear of the Wraith, who return periodically to cull their human herds. After taking their fill, the Wraith hibernate for centuries, watched over by Keepers, before they wake and feed again.
