The bigger definition of gibah(backbiting)

I am not still sure what prophet(pbuh) meant by using backbiting(gibah). According to this hadith => the Messenger of Allah (peace and blessings of Allah be upon him) said: “Do you know what ghibah (backbiting) is?” They said, “Allah and His Messenger know best.” He said, “Saying something about your brother that he dislikes.” It was said, “What if what I say about my brother is true?” He said, “If what you say is true then you have backbited about him, and if it is not true, then you have slandered him.” [Muslim] and many more, it seems to me that if anyone have done a bad work and I tell other about that to be careful from the person, then this would be backbiting as the person will be sad hearing that. But can't we make other people cautious of strange and bad persons. Or,I am wrong understanding The concept of backbiting? Then please make me clear about backbiting in islam. I would like to hear from bothsunni and shia perspective (i.e.from both sunni and shia hadiths) Thanks in advance