DDoS means distributed Denial of Service
With a DDoS attack, a lot of computers (often from different places around the world) make a lot of connection requests to a server. Because that server then receives as many requests in one time, the server (and therefore website) is often slow, or no longer reachable at all. To achieve this, botnets are often used. This way of attacking a company or website is increasingly being used to make the displeasure about a particular service or company clear.
Denial Of service attack [YouTube] HQagLcgUupQ [/youtube]
Visualization of a DDOS attack [YouTube] hNjdBSoIa8k [/youtube]
DDOS Infographic (with click through to YourHosting) 