Ethereal beings are insubstantial creatures which can move through solid objects. They therefore suffer no movement penalties for moving over difficult terrain or obstacles, or from other reductions to movement (such as the Anvil of Doom'sor the spell). They can move through buildings just as easily, but not through other units. They cannot, however, end their movement inside impassable terrain nor see through anything that would block the line of sight of normal units. Ethereal creatures block line of sight normally (the enemy cannot see through them).
Because they are insubstantial, they can't be harmed in combat except by attacks from magical weapons, Daemons or other Ethereal creatures, though they can be affected by spells as normal. Ethereal creatures can still be beaten in close combat and suffer extra wounds as a consequence because combat results are not wholly dependent upon casualties.
Note that if a Wraith rides a Nightmare, then the Nightmare will also be Ethereal.