They are reptilian in appearance; like most reptiles they "flick" their tongues to taste the air, indicating the presence of a Jacobson's organ (or analogue thereof). Their tooth shape indicates a strictly-carnivorous diet.
Some time ago, the star that the Veil's home planet orbited died and their planet along with it; however one Veil was off-world at the time and returned to find his home and species decimated. His name was Androvax and he took the destruction of his race very badly, with the event almost driving him mad. Androvax took it upon himself to become the force or will of the universe and travelled the galaxy(s), destroying planets, earning the name "Androvax the Annihilator ".
Despite there being only one adult member of their species left, a colony ship containing 100 Veil young in some sort of suspended animation was secretly impounded on Earth by the Alliance of Shades. Androvax learned of the location and (with the aid of Sarah Jane Smith, albeit reluctantly) stole it, piloting the vessel to find a new home world and rebuild his race.