JD,
DDOS = Distributed Denial Of Service
It usually means that a hacker, or group of hackers, got control of 10,000 or 1,000,000 or 100,000,000 computers and commanded all of them to visit a specific website all at once. This overloads the servers that are hosting the website, often causing them to crash. Unfortunately, if the server is hosting more sites than just the website that was attacked, the attack will bring all of those websites down until the attack stops and the server is brought back online.
There are other ways to execute a DDOS attack, but I believe this is the most common.