I understand DDoS is a DoS assault from multiple sources, where a coordinated stream of requests is launched against a target.
How is DRDoS different?
To put it straight and simple, DDOS is is making the server unavailable or denying the service to the users for a particular time.
DRDOS is sending forged requests from a target (victim) to millions of computers and making the target flooded by the responses from those computers.
EDIT (More info on DRDOS):
The attacker sends the requests from his server. Using IP address spoofing, the source address is set to that of the targeted victim, which means all the replies will go to (and flood) the target.