I think that everyone has a slightly different definition... and it is just a matter of picking what works for you.

For me, I consider an M/s relationship to be one that is 24/7 total power exchange ("TPE"). There is no switching in and out of roles. I consider a D/s relationship to be anything less than TPE... i.e. the partners are in their roles something less than 24/7... could be just in the bedroom, weekends-only, or just from time to time.

That is how I view it. I personally consider myself a slave... but I didn't start out that way. For some it is a natural transition to go sub --> slave, or Dominant --> Master, while others never have a desire to redefine their roles. For some Doms they feel that they can't be called a "Master" until they learn and perfect certain skills. But this isn't a hard and fast rule.

As for you question about slaves having different needs than subs... well, i think all slaves and all subs have different needs from each other. It all depends on the person. It isn't a case of "subs need x, and slaves need y." It all depends on needs, wants, desires, fears, experiences, challenges, etc.

So.. I hope I added some much needed cloudiness!