In a denial-of-service attack, a malicious party barrages a server with so many requests that it can’t keep up, or causes it to reset. As a result, legitimate users can only access the server very slowly — or not at all, as appears to be the case here.
Not only was the site down, but client applications that depend on the Twitter API could also not connect to the service, creating a complete Twitter blackout. According to June ComScore numbers Twitter has more than 44 million registered users and its user base has been growing rapidly for months as it becomes better known in the mainstream.
I know some of you were hyperventilating and all… but don’t worry, it’s back up.