What are psychological defense mechanisms and what are they defending?

Defense mechamisms are ways that we protect ourselves. Maybe your defense mechanism is to go silent when you feel threatened. Someone else's could be to attack when threatened. They are defending our egos, our emotions... sometimes our memories or self-esteem. Anything that we feel is threatening us Psychologically. Sometimes we defend against things that can't or won't hurt us, just because we worry that they might. That is why some people can't sustain a relationship... they dump anyone who gets too close to them. That's an overdeveloped protective response. Usually it is based in the fear of rejection: this person can hurt me, and I don't want to be hurt, so in order to avoid rejection, I am going to reject. There are many, many different defense mechanisms, but that is one example.