DDoS stands for Distributed Denial of Service. It is when a criminal forces a server running a website , or web based application, off line. They do this by sending so many requests to open pages or files to that web server that eventually the server can not cope, grinds to a halt and crashes - taking the website / app offline.
To run a DDoS attack needs a lot of computers all doing the same thing. To make that happen, the criminals infect a lot of computers and other internet capable devices with malware that turns each one into a bot (or zombie). The attacker then has remote control over the group of bots, which is called a botnet.
Once a botnet has been created, the attacker instructs the machine (bot) to send requests to the target site / server which often leads to it being overwhelmed and crashing - resulting in a denial-of-service to normal traffic (which is where the name comes from).
To run a DDoS attack takes very little skill as botnets already exisit and are available to hire from other criminals. A DDoS attack on the CIA's website was once described as the equivalent of taking down a poster put up by the CIA.
What is A DDoS Attack
Large companies survive DDoS attacks by having lots of web server capacity, and by hiring in extra when they are under attack. Companies like youtube who have to keep their site up all the time, and are likely to be targeted also switch between a number of different IP addresses for their sites to stop such attacks working for very long.