Freud believed that personality develops through five stages of psychosexual development - oral, anal, phallic, latency, and genital. Each stage is focused on a different erogenous zone. Fixation occurs if a child's needs are under- or over-gratified at a stage, impacting their adult personality. The phallic stage triggers the Oedipus complex where children desire the opposite-sex parent. Resolving this allows progression to the latency stage where sexuality is repressed until puberty's genital stage. Unresolved earlier issues can resurface and impact relationships.