Unlike Facebook, Twitter is asymmetrical. Sometimes you follow someone else’s twits and they follow yours as well. Sometimes it’s just one way. This Friend or Follow site is a cool little thing that shows you who follow that follows you as well ("friend") and who you follow that doesn’t follow you back ("following") and finally who follows you that you don’t reciprocate with ("fan").
I’m still unsure about the value of twitter so I experiment from time to time myself on the mix of who I’m following but if you want to follow me (i tweet comic stuff but other nonsense as well) go for it!