There are four letters which will cause dread in practically any network administrator: DDOS, or a Distributed Denial-of-Service attack. It's one of the most common forms of malicious attack against websites and other online entities, and also one of the hardest to defend against.
The basic theory behind a DDOS is quite simple: Get a whole bunch of computers to send a huge number of requests simultaneously to an online server. Since any server is going to have a maximum threshold for the number of connection requests it can serve at once, the DDOS overwhelms it and prevents it from doing its normal job. Or else it simply sucks up all the bandwidth before employees and customers can use it.