They are huge, huge, huge statues carved in the stone on the banks of the Anduin, made in the likenesses of Anarion and Isildur, Elendil's sons. Sort of like a cross between Mt. Rushmore and the Statue of Liberty (which won't mean much to you if you aren't from the US, I suppose).
Anyway, you should check out the Fellowship of the Ring movie. The scene with the Argonath is pretty impressive.
Look at this poster:
http://www.allposters.com/gallery.as...89&item=334264
[ December 10, 2002: Message edited by: Legalos ]