Is mentioning someone negatively for what they are, in order to advise those close to me to stay away from them, considered backbiting? Or when I am asked about them and I mention the bad things they did to me? Or criticizing others in their absence, such as criticizing politicians?

Backbiting is forbidden, but it is permissible out of religious necessity. This includes: seeking redress, identification, warning, disclosing open wickedness, for the one seeking a fatwa (definition), and for one who seeks assistance in removing an evil. It is permissible to mention a person's faults with the intention of advising others, or to mention their wrongdoing against you. As for criticizing others in their absence without a legitimate religious benefit, it is either backbiting (if what is mentioned about them is true) or slander (if it is not true), and there is no difference in this regard between a politician and anyone else.