A DDoS attack is most often a “bandwidth consumption” attack where the combined Internet connection bandwidth of many machines is “focused”, or directed, upon one or a few machines. Although the attack’s individual Internet packets may be completely harmless, a flood of such packets can overwhelm the target machine’s Internet connection or other packet-processing resources. The result is that valid traffic, unable to compete with the malicious flood, has little chance of obtaining useful service.
The attack seems to have been neutralized now, but I guess their will still be a lot of clean up to do for the next few hours. More info at the status page.