The concept comes from Sigmund Freud and was systematised by Anna Freud. The contribution that has held up best under research is George Vaillant's hierarchy, built on longitudinal studies spanning more than seventy years, which orders defences by how adaptive they are.
Mature: sublimation (difficult energy channelled into something productive), humour, altruism, anticipation. Vaillant found that using them predicts physical health and life satisfaction decades later.
Neurotic: intellectualisation (talking about pain as a concept), rationalisation, repression, reaction formation. They work, but they cost energy and hold experience at a distance.
Immature: projection, denial, acting out, passive aggression. They lower anxiety in the short term and typically produce precisely the relationship problems the person complains about.
What matters therapeutically: defences are not dismantled by confrontation. They are protecting something, and stripping them away before the person has anything to replace them with leaves someone exposed — which is why psychodynamic therapies work with them gradually and at the patient's pace.