Borderline personality disorder is one of the ten currently recognized personality disorders in the DSM-IV-TR. Each personality disorder, including BPD, has its own criteria for symptoms or characteristics. YOu can find more information about personality disorders here: http://psychcentral.com/disorders/#person or specifically about BPD: http://psychcentral.com/disorders/sx10.htm

The name has always been confusing. At first it reflected the idea that people with borderline personality disorder were somewhere on the border in between psychosis and neurosis. Most of the personality disorders resemble one or more Axis I disorders (which include psychotic disorders, as well as anxiety, depression, dissociative disorders, etc.) in some way. The main differences are often that the Axis I disorder may be more acute and the personality disorder more chronic. People who have Axis I disorders tend to be more distressed by their symptoms, while with personality disorders it's more like part of what the person's personality is like, and it may not occur to the patient that there is anything wrong with that. For example, people with Obsessive-Compulsive disorder have obsessions and compulsions that they feel compelled to act on or else they feel miserable and can't stop thinking about it. Those with Obsessive-Compulsive Personality Disorder have a rigid and compulsive way of doing things - they may make a lot of schedules and be perfectionistic, but it's not necessarily troublesome to the patient. Such a person might be difficult to be around a lot though, because they insist on doing things in a certain way and sometimes that isn't very fun.

BPD has features of several Axis I disorders, such as Depression, Bipolar disorder, PTSD, paranoia, etc. To further complicate things, people with personality disorders often have Axis I disorders too, so many people with BPD actually have Depression, PTSD, etc. Sometimes the Axis I disorder is treated and the acute symptoms improve, but the person still has all the BPD symptoms because their personality incorporates the tendencies to dissociate and have mood swings, etc.

I hope that I haven't just confused you more. It isn't an exact science by any means, and diagnostic categories aren't chiseled in stone anywhere either. In the future, the experts might decide that dysthymia (long-term mild depression) is actually a personality disorder, or they could decide that BPD fits better in an Axis I category and isn't a personality disorder afterall. Especially in the personality disorders, they keep changing their minds. Where I work, many of the clients last had psych evals 20 years ago or more, and have diagnoses that don't exist now, such as "Immature Personality Disorder." How would that be, to have a dx that someone decides is not actually a disorder?