Borderline Personality Disorder is a psychological imbalance defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM IV-TR), and is also called Emotionall Unsatable Personality Disorder, Borderline Type.
An exact definition can only be understood by someone with at least a masters degree in psychology. A good start is this Wikipedia link: http://en.wikipedia.org/wiki/Borderline_personalit...
If you are not a psychologist or psychiatrist, using these terms (on yourself, or about others) can be harmful. Please be careful. At the same time, if you know someone with the diagnosis, some understanding can be useful.
Someone with a borderline disorder can function in society, but their relationships are often harmful to themselves and others. Putting some of the psychological lingo carefully into plain English, people with Borderline personality disorder tend to:
1. make frantic efforts to protect themselves from being abandoned, whether or not they might actually be abandoned.
2. Idealize other people, thinking they are wonderful, then switching and deeply devaluing them.
3. Have intense, short episodes of irritability or anxiety, from a few hours to a few days long.
People with this disorder may tend to enter therapy and appear cooperative, but then run away when a real issue is touched, often jumping to a different therapist. In relationships, they can be both manipulative and clinging, which can feel very hurtful to others.
I hope this helps. If you are facing this disorder in yourself or someone you know, I recommend you read and learn and think. Even if you are not the one with the diagnosis, talking to an experienced therapist about ways of living with people with Borderline Personality Disorder can be helpful.