Unix is not a free OS like Linux(es) is(/are). Linux is a free clone of Unix written from scratch by Linus Torvalds and some hackers (basically
heh). Unix has it's own website where you will find more info...Linux was made to be as much the same as Unix as possible, but free, and from scratch (not a piece of original code was used as far as I understand).
read Unix's website for more info on this....Linux has distros because it's free, but Unix doesn't as far as I know, because it is not.