DDOS attacks are distributed denial of service attacks. You may have heard about them in the news lately as they were used to bring down websites in the northeastern USA and other areas recent in a broad attack.
DOS (denial of service) attacks have been around a long time and involved having a computer send a server a bunch of requests for information that have a high priority that stopped the server from being able to respond to normal items such as requests for the web page. Most firewalls will stop them as it is lots of requests from one address and easy to spot.
DDOS involves lots of requests for information to a server coming from lots of locations. Usually they do this by putting Trojan or related viruses on machines with directions in the Trojan to begin sending out the messages at a specific time. Anti-virus software will prevent these programs getting on a machine and it being used as an attacker. In the recent case they found a new weakness in our Internet infrastructure. They used the Internet of Things (or Everything) devices which have not been securely built for security to do the attack. This includes such things as internet enabled appliances, home security systems (which gives you the ability to lock door or windows when not at home) and lots of others to be the attackers.
It is apparent from this that builders of Internet enabled devices will need to start building in security and also for ways for the devices to be updated so they can be protected from malware and being used by third parties in malicious ways