Most users have regained access through the http://twitter.com site, however many apps and clients which use the API are expected to experience some delay for the next several hours.

Denial of Service (DoS) attacks are caused by a malicious party flooding the target network with requests, making it difficult or impossible for normal network transactions to occur.

Latest info on the attack points to a Distributed Denial of Service (DDoS) which uses hundreds or thousands of computers under control of an attacker. These computers are usually infected by spyware/trojans which carry the attack software.

-Todd

(from CNN.com) http://bit.ly/PeNWm

Also from CNET.com http://bit.ly/19SCux