It's a mental illness. According to the American Psychiatric Association, it is "an enduring pattern of inner experience and behavior that deviates markedly from the expectations of the culture of the individual who exhibits it"
In other words it is a constant factor in a person's behaviour and thought patterns, which affect her ability to interact in a given society, and which make her different to the extent that it could bother her and her surroundings.
The DSM-IV lists ten personality disorders, grouped into three clusters.
Cluster A (odd or eccentric disorders)
Cluster B (dramatic, emotional, or erratic disorders)
Cluster C (anxious or fearful disorders)
