A personality disorder is defined as a pervasive pattern of "inner experience and behavior" that is deviant from a person's cultural norms. These may be deviations in thoughts, emotionality, interpersonal relatedness, and impulse control. Deviations in any of the above aspects need to be pervasive, stable, present at least since adolescence, and not due to substances or another mental disorder. Importantly, these ways of thinking, feeling, or behaving need to be significantly distressful and problematic.
 

The second aspect involves defining what type of personality disorder is present. DSM-IV currently lists ten: paranoid, schizoid, schizotypal, narcissistic, antisocial, borderline, histrionic, avoidant, dependent, obsessive-compulsive, with a catch-all "not otherwise specified category". Each personality disorder had a certain number of criteria, to which you must meet an artificial cut-off. So to be Borderline, for example, you need to have five symptoms out of nine possible symptoms such as: self-harming, unstable relationships, fear of real/imagined abandonment, impulsivity, identity disturbance, etc.

*This information is credited to the American Psychiatric Association.