Borderline Personality Disorder is a psychiatric diagnosis. Basically there are three types of psychiatric disorders. There is a "thought" disorder. There is a "mood" disorder. There is the "personality" disorder.
Thought disorders deal with a person's ability to correctly internalize and understand what is happening around them as well as inside them and the relationships between the two. A thought disorder shows itself in anything from confusion and inability to correctly understand environmental cues to inability to control thoughts to determine what is reality and what is fantasy. Thought disorders often have "positive" symptoms which mean that something is added to the person that is not present in most people (some examples are hallucinations or delusions). Thought disorders often also have "negative" symptoms which means something is taken from the person that is present in most people (for example the ability to show emotions in body language including facial cues and/or the inability to feel emotions).
Mood disorders are a disruption of a persons moods/emotions. This most often shows itself in depression that is disproportionate to any stimuli or not connected to any stimuli at all and/or lasts for extended periods. This may also show itself in having extreme heightened excitement (mania) not connected to any environmental stimuli or disproportionate to the stimuli that is present.
Personality Disorders, also sometimes called character disorders, are significantly divergent personalities from the norm. This refers to the very core of what the person considers to be themselves. This class of psychiatric disorder is often considered to be the most difficult to treat/change, especially because there is not any medication known to have a positive impact on this type of disorders.
Borderline Personality Disorder indicates a personality that is on the borderline so to speak and crosses over back and forth for what is considered normal. It often shows itself as difficulty in maintaining relationships although typically the ability to create relationships is not impaired. It can often be noticed as a person who is unable to determine what makes up themselves (shown as extreme lack of consistency in action, attitude or behavior). It often also shows in the persons inability to managing their own feelings and/or moods such that they overreact through their behaviors, attitudes or actions. They often do a variety of actions that are typical and are thought by some to be a concrete symptoms of this disorder: I am speaking of self injurious behavior (i.e. cutting oneself) or frequently threatening and or attempting suicide. Frequent attempts at manipulation of those around the person and attempts to play one person against another to divert the attention from oneself when things go awry are prevalent as well.
So having said that as preliminary I ran across this blog that is graphic (swear words, and talk of suicide) that you may want to view as an example of this disorder (almost a text book example) and give you a view of what this feels like for the individual. This is not a person I am in anyway acquainted with myself. Click here to view. The comments to this blog are of interest as well.