What is BPD?

The main feature of borderline personality disorder (BPD) is a pervasive pattern of instability in interpersonal relationships, self-image and emotions. People with borderline personality disorder are also usually very impulsive, oftentimes demonstrating self-injurious behaviors (risky sexual behaviors, cutting, suicide attempts).
Borderline personality disorder occurs in most by early adulthood. The unstable pattern of interacting with others has persisted for years and is usually closely related to the person’s self-image and early social interactions. The pattern is present in a variety of settings (e.g., not just at work or home) and often is accompanied by a similar lability(fluctuating back and forth, sometimes in a quick manner) in a person’s emotions and feelings.
These individuals are very sensitive to environmental circumstances. The perception of impending separation or rejection, or the loss of external structure, can lead to profound changes in self-image, affect, cognition, and behavior. They experience intense abandonment fears and inappropriate anger, even when faced with a realistic time-limited separation or when there are unavoidable changes in plans (e.g., sudden despair in reaction to a clinician’s announcing the end of the hour; panic or fury when someone important to them is just a few minutes late or must cancel an appointment). They may believe that this “abandonment” implies they are “bad.” These abandonment fears are related to an intolerance of being alone and a need to have other people with them. Relationships and the person’s emotions may sometimes be seen by others or characterized as being shallow.
A personality disorder is an enduring pattern of inner experience and behavior that deviates from the norm of the individual’s culture. The pattern is seen in two or more of the following areas: cognition; affect; interpersonal functioning; or impulse control. The enduring pattern is inflexible and pervasive across a broad range of personal and social situations. It typically leads to significant distress or impairment in social, work or other areas of functioning. The pattern is stable and of long duration, and its onset can be traced back to early adulthood or adolescence.

A person with this disorder will also often exhibit impulsive behaviors and have a majority of the following symptoms:

• Frantic efforts to avoid real or imagined abandonment

• A pattern of unstable and intense interpersonal relationships characterized by alternating between extremes of idealization and devaluation

• Identity disturbance, such as a significant and persistent unstable self-image or sense of self

• Impulsivity in at least two areas that are potentially self-damaging (e.g., spending, sex, substance abuse, reckless driving, binge eating)

• Recurrent suicidal behavior, gestures, or threats, or self-mutilating behavior

• Emotional instability due to significant reactivity of mood (e.g., intense episodic dysphoria, irritability, or anxiety usually lasting a few hours and only rarely more than a few days)

• Chronic feelings of emptiness

• Inappropriate, intense anger or difficulty controlling anger (e.g., frequent displays of temper, constant anger, recurrent physical fights)

• Transient, stress-related paranoid thoughts or severe dissociative symptoms

Because personality disorders describe long-standing and enduring patterns of behavior, they are most often diagnosed in adulthood. It is uncommon for them to be diagnosed in childhood or adolescence, because a child or teen is under constant development, personality changes and maturation. However, if it is diagnosed in a child or teen, the features must have been present for at least 1 year.
Borderline personality disorder is more prevalent in females (75 percent of diagnoses made are in females). It is thought that borderline personality disorder affects between 1.6 and 5.9 percent of the general population.
Like most personality disorders, borderline  personality disorder typically will decrease in intensity with age, with many people experiencing few of the most extreme symptoms by the time they are in the 40s or 50s.

Taken from http://psychcentral.com/disorders/borderline-personality-disorder-symptoms/