You need to authenticate with Twitter to use this site.
This app will find the intersection between your followers and those of another person. It's handy for those times when you want an idea of how many people will see an @reply, or if everyone who's likely to care will already have seen that witty bon mot you were planning to retweet.
It's different from almost every other app like this that I've seen in that it's written from the start to use Twitter authentication, which means that people with private accounts (like, er, me) can use it.
I hope you find it useful.