is a tool that allows you to compare the friends or followers of two or more Twitter users to see who they have in common. For example, a common use for TweepDiff is to compare your own friends and followers to see who you follow that doesn't follow you back or vice versa. Another use is to compare who you follow with someone else to see if they know of people that may be interesting to you.
for more details about the functionality of TweepDiff and how to use it.
You can build TweepDiff URLs without using the interface. The URLs have the format
is the screen name of the Twitter user and
is either 'friends' or 'followers' (or fr/fo for short).