"Lowers the defense and resistance by 30%"
^ can't confirm this...just what others said in the forums.

Tag = mark
can be cured off(since it's a debuff)

Konan and Hidan can hit all Tagged targets with their mystery skill.
I believe Minato(not yet unlocked) c*so hit more targets if he uses his mystery on someone that has a tag.

Not sure if there are others :D