Mibs, when they took the servers down they gave no information at all. Everyone was kept in the dark about it. Not even a "we're experiencing technical difficulties" message. This went on for days with no answer and then they suddenly drop a bombshell of "our servers have been compromised, we don't know the full extent of it but we know that our customers usernames/passwords, birthdays, e-mail and postal addresses have been compromised and there is a possibility that credit cards have been compromised.". Sony has a good security system set up, but they weren't using it as extensively as they should have for several million people's information, especially with the Anonymous attacks over the past month.
By the way, here is Anonymous' message to Sony
regarding the attacks on the servers.
That being said, a DDoS attack is when a hacker, or group of hackers, pings a server/field of servers and overwhelms them to the point the servers shut down to avoid damage. A ping is a signal sent to the target that the target must decode and process. Kinda like when a cable company checks to see if your cable box or modem is working properly. DDoSing is sending multiple pings at the same time in a rapid fire manner. The servers, on top of handling normal pings from the clients (players) can't handle the massive influx of signals and will shut down temporarily. When they're brought back up, the hackers (Anonymous) will do the attack again. Thus the acronym DDoS, or Distributed Denial of Service. As I mentioned, they did this to the former Egyptian government and managed to shut down government websites that stayed down until Muburak's resignation.
They also DDoS'd, prank called, and black faxed (sent faxes that were solid black sheets to kill the fax machine's ink) the Church of Scientology back in 2008 for removing a leaked video from youtube that had Tom Cruise saying that scientologists would be the only ones that would help at the scene of a car crash; that Scientologists were the world's authority on reforming criminals and getting people off drugs, among other things. The video Anonymous sent is actually pretty cool.
Now that you have an idea of what DDoSing is, what happened this time wasn't DDoS. This latest outage is due to someone actually breaking through Sony's security about their user database and downloading information of all Sony's customers on the PSN. While it's not Anonymous' style (they don't try to steal from people, just harass), the timing is too convenient with their attacks on Sony. They're goal was to shut Sony's PSN system down to get Sony to stop the legal action against geohot, a hacker that "jailbroke" the ps3 (bypassed the security in his ps3 to unlock the ability for it to be used as a computer again) then posted the method online for others to jailbreak their systems.