A DDoS attack is a more advanced and more dangerous form of DoS flooding attack. Rather than use a single source to attack the target, DDoS uses multiple synchronized launch sites.
A Distributed Denial of Service (DDoS) attack uses multiple computers to launch a simultaneous coordinated DoS attack against one or more targets. The process involves compromising multiple servers and installing a single master program on one system and as many agent programs on other computers as possible. The master program is then used to instruct the agent programs to launch simulataneous DoS attacks against the target or targets. The effect is not simply a massive DoS attack, it is also an anonymous attack since the agents' host computers are not aware they are being used and the initial instigator cannot (easily) be traced.
There is no simple way to protect yourself against DDoS attacks since you are reliant upon other computers not being compromised. The best you can do is make sure that your own systems are fully patched and that you are not vulnerable to being used as a launchpad yourself. If everybody did this, then DDoS attacks would be less of a problem than they are. However, you need to do this for reasons beyond altruism: there is an increasing likelihood that you could be held liable under tort for any damage that you even unwittingly and unknowingly cause to a third party. In other words, if you do not take adequate action to prevent your system being compromised and used in a DDoS attack, you could be held liable for damages incurred by the target.
A variation on DDoS emerged during 2001 in which the target appears to be the Internet itself rather than a specific domain. This approach tends to use viral dissemination techniques to compromise multiple servers. The 'virus' then lays dormant until a pre-determined point -- at which time all of the compromised machines attempt to promulgate. If enough hosts have been compromised, the entire Internet can be affected. Code Red was perhaps the first serious example of this type of DDoS.