In my oppinion, a clone is identical in most repects i.e. both shape and mechanism are identical, as are all design apects. The only difference in a clone are the quality of the materials used and the markings.
A rip-off simply has the same general design, but differs in many other aspects, i.e. the blade can be a slightly different shape, as well as the handles, the colour can be way different etc.
Bear in mind though that these are not legal terms. Even a knife that is clearly different from the original but uses identifiable design aspects will often be considered to be a copyright infringement.
As for where that particular line is drawn, well, lawyers have made a lot of money arguing such cases back and forth...