Twitter, like most social networks, is as good as you make it. But you can't always know if a particular user is going to be worth following until you do. Trolldor helps out by reporting on behaviour that might make a user troll-esque.
Spammers, trolls and other internet undesirables tend to have some common traits. On Twitter, this includes following too many accounts, only ever mentioning others, or getting a majority of content from other people's accounts. Trolldor provides stats on these so you don't have to wade through their feed to see if they will be worth your time. You can also report abusive users to Trolldor if they're particularly obnoxious. Trolldor can't get anyone banned, but then some behaviour isn't necessarily against Twitter's policies. It's just really obnoxious.