I'll just paste the wiki article, that should be enough for the basis of a discussion.
Paranoid personality disorder is a psychiatric diagnosis characterized by paranoia and a pervasive, long-standing suspiciousness and generalized mistrust of others. (DSM-IV)
For a person's personality to be considered a personality disorder, an enduring pattern of characteristic maladaptive behaviors, thinking and personality traits must be present from the onset of adolescence or early adulthood. Additionally, these behaviors, traits and thinking must be present to the extent that they cause significant difficulties in relationships, employment and other facets of functioning.
Those with paranoid personality disorder are hypersensitive, are easily slighted, and habitually relate to the world by vigilant scanning of the environment for clues or suggestions to validate their prejudicial ideas or biases. They tend to be guarded and suspicious and have quite constricted emotional lives. Their incapacity for meaningful emotional involvement and the general pattern of isolated withdrawal often lend a quality of schizoid isolation to their life experience. 
* Because of the surface similarities of the paranoia involved, it is important that the paranoid personality disorder not be confused with paranoid schizophrenia, another totally different type of mental disorder where the patient has constant feelings of being watched, followed or persecuted.
Descriptive diagnosis per American DSM-IV-TR
Paranoid personality disorder is listed in the DSM-IV-TR as 301.00 Paranoid Personality Disorder.
According to the DSM-IV-TR, this disorder is characterized by a pervasive distrust and suspicion of others such that their motives are interpreted as malevolent, beginning by early adulthood and present in a variety of contexts, as indicated by four (or more) of the following:
* Suspects, without sufficient basis, that others are exploiting, harming, or deceiving him or her
* Is preoccupied with unjustified doubts about the loyalty or trustworthiness of friends or associates
* Is reluctant to confide in others because of unwarranted fear that the information will be used maliciously against him or her
* Reads benign remarks or events as threatening or demeaning.
* Persistently bears grudges, i.e., is unforgiving of insults, injuries, or slights
* Perceives attacks on his or her character or reputation that are not apparent to others and is quick to react angrily or to counterattack
* Has recurrent suspicions, without justification, regarding fidelity of spouse or sexual partner.
The traits, behaviors and characteristics
* Do not occur exclusively during the course of a mood disorder accompanied by psychotic features nor other psychotic disorders.
* Are not due to the direct physiological effects of a general medical condition.
 International description per ICD-10
The ICD-10 lists paranoid personality disorder as F60.0 Paranoid Personality Disorder.
This personality disorder is characterized by at least 3 of the following:
(a) excessive sensitivity to setbacks and rebuffs;
(b) tendency to bear grudges persistently, i.e. refusal to forgive insults and injuries or slights;
(c) suspiciousness and a pervasive tendency to distort experience by misconstruing the neutral or friendly actions of others as hostile or contemptuous;
(d) a combative and tenacious sense of personal rights out of keeping with the actual situation;
(e) recurrent suspicions, without justification, regarding sexual fidelity of spouse or sexual partner;
(f) tendency to experience excessive self-importance, manifest in a persistent self-referential attitude;
(g) preoccupation with unsubstantiated "conspiratorial" explanations of events both immediate to the patient and in the world at large.
* expansive paranoid, fanatic, querulant and sensitive paranoid personality (disorder)
* delusional disorder
A personality disorder is a severe disturbance in the characterological constitution and behavioral tendencies of the individual, usually involving several areas of the personality, and nearly always associated with considerable personal and social disruption. Personality disorder tends to appear in late childhood or adolescence and continues to be manifest into adulthood.
It is therefore unlikely that the diagnosis of personality disorder will be appropriate before the age of 16 or 17 years. General diagnostic guidelines applying to all personality disorders are presented below; supplementary descriptions are provided with each of the subtypes.
 Diagnostic Guidelines
Conditions not directly attributable to gross brain damage or disease, or to another psychiatric disorder, meeting the following criteria:
(a) markedly disharmonious attitudes and behavior, involving usually several areas of functioning, e.g. affectivity, arousal, impulse control, ways of perceiving and thinking, and style of relating to others; (b) the abnormal behavior pattern is enduring, of long standing, and not limited to episodes of mental illness; (c) the abnormal behavior pattern is pervasive and clearly maladaptive to a broad range of personal and social situations; (d) the above manifestations always appear during childhood or adolescence and continue into adulthood; (e) the disorder leads to considerable personal distress but this may only become apparent late in its course; (f) the disorder is usually, but not invariably, associated with significant problems in occupational and social performance.
ICD-10 copyright © 1992 by World Health Organization.