Just a few while ago, Twitter has been exploited by a very notorious hacker. Twitter has been prone to hacks and viruses as well as other social networking sites like Facebook, Myspace and many other more. This is because the greater impact it will have on the social media chaos, the greater the hackers think their achievement is.
What the virus was doing is to retweet malicious sites to your friends and when you hover the mouse on it, some third party pop-up website will appear and will install malicious applications into your computer.
As for now, Twitter status update message said that they stabilized the situation already. You can check that status update here.