CLAMP is a four-woman group whom have worked on a large number of popular shoujo comics such as the ever fluffy Card Captor Sakura, the dark and brooding Tokyo Babylon, the violent and apocalyptic X/1999, the popular Magic Knight Rayearth, and many others.
RG Veda is CLAMP's debut work (or so it has been said). Before RG Veda, which was published as a serial in "Wings"; they were primarily "Doujinshi" artists, or fan manga artists. RG Veda is a rather old manga/anime, and it is unfortunate that the manga for it is quite rare. The manga is infinitely superior to the movies due to the fact that much in the anime was squashed into two OAVs and there was no ending to them. The manga was translated into french, chinese and german(?). If any of the information on CLAMP or RG Veda is incorrect, please point it out to me. The info will be very much appreciated ^-^.