yeah, i didn't really feel like getting into it, but i completely agree..
clone only is simple, it's a female plant that has no male around to make seeds of it.. a lot of times it can simply be an elite pheno of a strain, say a one in a 10000 kind of thing, or it can be the result of a hermie cross, like sour diesel is rumored to be, or it could be the result of a normal cross where the parents are no longer around to make the cross again..
you can clone a clone of a clone of a clone of a clone of a clone, etc, pretty much on and on and on and on, and it will be an identical copy of the parent, like potroast said, hence the term clone.. they do not lose vigor, etc.. you might get different expressions from different environments though..
if clones lost vigor over time, clone only's wouldn't be so sought after..