In general, people with BPD feel worthless, empty, moody, needy, depressed, and have difficulty managing their emotions. They have trouble with boundaries--both their own and respecting others. They are looking for that all-loving "other" who will provide the unconditional love they cannot give themselves.
BPs often act inconsistently, act impulsively in ways they later regret, see other as either all good or all bad, and base their beliefs on feelings instead of facts. Borderlines are people in pain.
BPs commonly manage their intense pain in two ways: they act in or act out. BPs who act in may mutilate themselves, make suicide attempts, express self-hate, or seek outpatient or inpatient therapy.
Other BPs manage their pain by trying to foist it on others. They blame loved ones for all their problems, criticize, make unfair accusations, act emotionally or physically abusive, put others in no-won situations, and use emotional blackmail to get the love they need.
A longer, clinical definition of BPD used by clinicians:
The DSM-IV diagnostic criteria for borderline personality disorder reads as follows:
A pervasive pattern of instability of interpersonal relationships, self-image, and affects moods, and marked impulsivity beginning by early adulthood and present in a variety of contexts, as indicated by five (or...