Twitter founder @ev has issued a warning to Twitter users over those dodgy-looking "get followers fast!" websites. He's advising people to avoid them, as they can leads to accounts getting "phished, hacked, or even suspended".
Like any other web service, as Twitter has gained in popularity, it's also become a target for spammers and others with ulterior motives. Many sites promise a bucketload of followers, but require you to input your username and password.
They then use that to spam all your followers, and can even change your password so that you're unable to access it any more yourself. The best way to stay safe on Twitter is not to put your password into any services that you don't trust 100%. Most should work with just a username, so if in doubt about a site, then don't use it.