Glamouring is a magical method of changing one's appearance used by Witches, Warlocks, Whitelighters and Elders. It is similar to, but not synonomous with, Shapeshifting. Glamouring casts an illusion around a being so the change is in appearance only, while shapeshifting physically transforms the being.
Glamouring can be activated by ability, potion or spell. The first time glamouring is shown is when Warlock Anton glamours in to former lover of Piper's past life. Also Leo Wyatt glamours into the demon Belthazor to train the sisters. In this type of glamouring the person manifests with bright blue orbs.
We see that Glamouring is used by Paige because she is half whitelighter. But we also see that Phoebe and Piper have this ability too ("Something Wicca This Way Goes"). This shows that all three of the sisters can glamour.