How it work:Like it is stated in a website:
FaceNiff is an Android app that allows you to sniff and intercept web session profiles over the WiFi that your mobile is connected to.
It is possible to hijack sessions only when WiFi is not using EAP, but it should work over any private networks (Open/WEP/WPA-PSK/WPA2-PSK) It’s kind of like Firesheep for android. Maybe a bit easier to use (and it works on WPA2!).
However, it doesnt work for all android device, it only works on Rooted Andoid Devices, therefore, please make sure your andoid has been rooted. If you still dont know what rootes android phone is, you can see here (Rooting Android Without PC ). Here are some website that are supported by Faceniff for android. They are: