Wights are dead men or creatures raised up by the Others, seemingly when touched by the cold that accompanies them.  Anyone who falls against the Others must be burned, or else the dead will rise again as their thralls. Fear of their own dead becoming wights leads the wildlings to burn them.
The appearance of wights depends entirely on the condition of the corpse when it is raised. Some are lifelike, while others are badly rotten although the process of decay has been halted. All are easily identified by their eyes having turned bright blue  like two blue stars  and their hands and feet black and swollen with pooled and congealed blood. 
Wights are supposedly attracted to warm blood and will attack with surprising strength. Unlike the Others they are slow and clumsy.  Wights have a queer, cold scent that can panic animals if they catch a wiff of it. 
It appears that wights retain at least some of their former memories, since the wight of Othor seemed to know where Lord Commander Jeor Mormont slept and who he was as it tried to kill him, and the wight of Thistle recognises Varamyr Sixskins even warged into the body of a wolf.
Being dead, wights feel no pain and will continue to fight regardless of injury.  Though they can be stopped by total dismemberment, their limbs will continue to move if detached from their bodies. When a wight is destroyed, the blue disappears from its eyes.  
Wights appear to be able to remain in a state of stasis, such as outside the hillside entrance to the cave of the three-eyed crow. The snows have buried most of the dead men, but they are still there, hidden, frozen, waiting. 
Dragonglass cannot stop a wight  but fire can. They are highly flammable and will be quickly consumed if set aflame. It is unknown at this time whether wights can cross the Wall on their own, although corpses brought through the Wall can apparently still reanimate as wights. Burning corpses appears to prevent them from being reanimated by the Others to become wights.
The Seven-Pointed Star
A list of individual characters that have been turned into wights over the course of the series:
A Game of Thrones
A Clash of Kings
Othor’s hand (still moving) was sent to King's Landing in a jar with Alliser Thorne to try to an impress on the king the severity of the threat from beyond the Wall so that the king may send more troops to the wall. Tyrion Lannister kept Ser Allister waiting so long that Othor's hand decomposed, and was not very convincing.
A Storm of Swords
During the Battle of the Fist of the First Men the Night's Watch was unable to defend against the wights that were sent against them. After a crushing defeat, the majority of the Night's Watch who fought on the Fist were slain by the insurmountable odds.
A Dance with Dragons
While listening to gossip in the Lazy Eel in White Harbor Davos notices that no one speaks of King Stannis. No one seems to know he has gone north to help defend the Wall. Wildlings and wights and giants had been all the talk of Eastwatch, but no one seems to be giving them so much as a thought.
Lord Commander Jon Snow states to his black brothers that his lord father used to tell him that a man must know his enemies. He states that they understand little of the wights and less about the Others. They need to learn.
Cotter Pyke sailed towards Hardhome by the command of Lord Commander Jon Snow. By the time Cotter arrived at Hardhome he sent a report to say there were dead things in the woods and dead things in the water. 
References and Notes
- A Game of Thrones, Chapter 52, Jon VII.
- A Game of Thrones, Prologue.
- A Storm of Swords, Chapter 33, Samwell II.
- A Storm of Swords, Chapter 46, Samwell III.
- A Dance with Dragons, Chapter 4, Bran I.
- A Storm of Swords, Chapter 18, Samwell I.
- A Dance with Dragons, Chapter 34, Bran III.
- A Feast for Crows, Chapter 27, Jaime III.
- A Game of Thrones, Chapter 52, Jon
- A Dance with Dragons, Jon VIII