Glamors make the object of the magic appear differently to observers, although the object does not actually change its shape. Glamors are strongest when built on a particular object or body part belonging to the person which the illusion will resemble. The object of the glamor wears an item to maintain the spell, such as a ruby or a moonstone. Sorcerers usually need to study for years before they are able to create a glamor. Some red priests believe that glamors are made possible through R'hllor, the Lord of Light.
A Feast for Crows
A Dance with Dragons
To spare Mance Rayder from being executed by Stannis, Melisandre glamors the King-Beyond-the-Wall. Mance wears a ruby in a fetter and bone armor to appear as the Lord of Bones, who is likewise glamored into appearing as Mance. Rattleshirt is thus burned in a cage, while Mance is sent on a mission to rescue "Arya" (actually Jeyne Poole) from House Bolton.
Mance: The glamor, aye. I feel it when I sleep. Warm against my skin, even through the iron. Soft as a woman's kiss. Your kiss. But sometimes in my dreams it starts to burn, and your lips turn into teeth. Every day I think how easy it would be to pry it out, and every day I don't. Must I wear the bloody bones as well?
Melisandre: The spell is made of shadow and suggestion. Men see what they expect to see.
Jon: What sorcery is this?
Melisandre: Call it what you will. Glamor, seeming, illusion. R'hllor is Lord of Light, Jon Snow, and it is given to his servants to weave with it, as others weave with thread.
The bones remember. The strongest glamors are built of such things. A dead man's boots, a hank of hair, a bag of fingerbones. With whispered words and prayer, a man's shadow can be drawn forth from such and draped about another like a cloak. The wearer's essence does not change, only his seeming.