How can one distinguish between backbiting (Gheebah) and insult (Sabb), and does the backbiter bear the sin of the one he backbit even if the latter's sin is related to grave sins (Fawāḥish)?

Slander is verbal abuse, and backbiting is mentioning something about your brother that he dislikes. Both are forbidden. Whoever backbites or wrongs someone will face retribution on the Day of Judgment. The wronged person will take from the oppressor's good deeds proportionate to the injustice. If the oppressor has no good deeds, some of the wronged person's bad deeds will be placed upon the oppressor, in proportion to the injustice. This is in accordance with the Prophet’s (peace be upon him) saying: "Whoever has wronged his brother concerning his honor or anything else, let him seek his forgiveness today, before there will be no dinar or dirham. If he has good deeds, they will be taken from him in proportion to his wrongdoing. And if he has no good deeds, some of the bad deeds of his companion will be taken and placed upon him."