RAC stands for reservation against cancellation. That means, if anybody with confirmed ticket cancels his/her ticket, you might get a berth.
Theoritically RAC and Waitlist (WL) are one and the same.
First few unconfirmed tickets are termed by IRCTC (Indian Railways) as RAC and being RAC, you are allowed to board the train, even if the ticket doesn't get confirmed at time of boarding. You are guaranteed a seat on shared basis. If you are lucky, you might a berth too.
WL e-tickets are cancelled automatically when the final-chart is prepared. So, you can't board the train if at least one of the passengers status is not confirmed.