What Is a DDoS Attack?
Before we can understand just how groundbreaking this recent attack was, let’s first go over exactly what a denial of service attack is. It is one of the least complicated attacks that a hacker can pull off. Basically the goal is to shut down a webserver or connection to the internet. Hackers accomplish this by flooding the server with an extremely large amount of traffic.

It would be like taking a wide open freeway and packing it full of the worst rush hour traffic you could imagine. Every connection to and from the freeway would grind to a halt. This would make visiting the website (or the road) next to impossible, or at the least extremely slow! In some cases, the server might overload and shut down completely.

When this happens, it doesn’t mean that the website was necessarily hacked. It just means that the website was kicked off the internet for a period of time. This may not sound like that big of a deal, but if your company relies heavily on its online presence, this interruption of service could take a huge cut out of profits.

DoS v. DDoS
The next item to be clarified is the difference between a DoS (Denial of Service) attack and a DDoS or (Distributed Denial of Service) attack. This distinction is pretty simple: a DoS attack comes from one network or computer whereas a DDoS comes from multiple computers or networks. DDoS attacks are most always bigger than a DoS attack because the strength of the attack can be multiplied by a huge amount of computers.

Source: http://www.scientificamerican.com/article.cfm?id=what-is-ddos-attack