Not necessarily mental illnesses per se, but personality disorders are psychological conditions that affect people and their relationships.
Personality disorders are defined by the American Psychiatric Association (APA) as “an enduring pattern of inner experience and behavior that deviates markedly from the expectations of the culture of the individual who exhibits it”. These patterns, as noted, are inflexible and pervasive across many situations, due in large part to the fact that such behavior is ego-syntonic (i.e., the patterns are consistent with the ego integrity of the individual), and therefore, perceived to be appropriate by that individual. The onset of these patterns of behavior can typically be traced back to late adolescence and the beginning of adulthood, and, in rare instances, childhood.
Similar Minds has a good summary of personality disorders:
The first group are the Eccentric Personality Disorders. These people often appear strange or peculiar to others.
- Paranoid Personality Disorder – individual generally tends to interpret the actions of others as threatening.
- Schizoid Personality Disorder – individual generally detached from social relationships, and shows a narrow range of emotional expression in various social settings.
- Schizotypal Personality Disorder – individual is uncomfortable in close relationships, has thought or perceptual distortions, and peculiarities of behavior.
The second group are the Dramatic Personality Disorders. These people have intense emotional mood swings and distorted perceptions of themselves and impulsive behaviours.
- Antisocial Personality Disorder – individual shows a pervasive disregard for, and violation of, the rights of others.
- Borderline Personality Disorder – individual shows a generalized pattern of instability in interpersonal relationships, self-image, and observable emotions, and significant impulsiveness.
- Histrionic Personality Disorder - individual often displays excessive emotionality and attention seeking in various contexts. They tend to overreact to other people, and are often perceived as shallow and self-centered.
- Narcissistic Personality Disorder – individual has a grandiose view of themselves, a need for admiration, and a lack of empathy that begins by early adulthood and is present in various situations. These individuals are very demanding in their relationships.
The third group are the Anxious Personality Disorders. These people are often fearful and anxious of one or many things.
- Avoidant Personality Disorder – individual is socially inhibited, feels inadequate, and is oversensitive to criticism
- Dependent Personality Disorder – individual shows an extreme need to be taken care of that leads to fears of separation, and passive and clinging behavior.
- Obsessive-Compulsive Personality Disorder – individual is preoccupied with orderliness, perfectionism, and control at the expense of flexibility, openness, and efficiency.
Similar minds also has a test for personality disorders you can take here.
Update: under the DSM-IV, some previously identified disorders were dropped from the 3 clusters above and remain under the classification: “Not Specified” or “Needing Further Study”. Find out more here.