I think the difference is mostly one of degree. Let me start with the general defintion and go from there. A sub has limits, a slave does not.

To me this simplistic because it implies a level of obedience that would be all but impossible to a self respecting person. A sub is someone that has decided that she wants to experience submitting to someone eles. This can be a limited type of submission, ie, "I wil do whatever you want me to tonight." Or a LTR with clearly defined limits, whatever they might be.

A slave is someone who is comfortable enough with the Dom that the limits are basically set by the Dom. In other words, a slave woud be someone who decides to obey a Dom no matter wht the circumstances because the level of trust is such that the slave and the Dom know each others limits and respect them.

I do know of slaves who will tell you that the Dom knows what is best, and that though that person would normally not perform a certain task, they would do so if told to by the Dom. As I said, this attitude is one that I would have some serious questions about.

I know this is an awkward explanation, but I hope it does clarify things for you. As a sub you have a safe word, but as a slave you give that word back to your Dom.