A Distributed Denial of Service (DDoS) attack attempts to disable a site or web service by sending a ton of requests from multiple sources. Essentially, the server buckles under the pressure. Sometimes this is done to silence sites that the attackers disagree with, or they might try to take advantage of business backends.
The Digital Attack Map, a collaboration between Google Ideas and Arbor Networks, shows current attacks and serves as a browser for past attacks around the world. Color and size indicate the type of attack and movement represents origins and destinations.