Pehaps I missed something, but in my first abstract algebra course we were taught that if in a ring either cancellation law (i.e. $ac = bc, c\neq 0 \Rightarrow a=b$; either this one or the left one), then said ring had no zero divisors. The converse holds too: if a ring has no zero divisors, both cancellation laws hold.
My first question is: Does this mean that any division ring has no zero divisors?; because, in a division ring, if one has $ac = bc$ with $c\neq 0$, one could always multiply both sides by $c^{-1}$ and $ac = bc$ would imply that $a=b$ and, thus, the right cancellation law would hold.
My second question is: Are there more necessary and sufficient conditions for a ring to have no zero divisors?