Temperament is the biological component of your personality -- it is the part of your personality that you are born with. Your temperament is determined largely by your genes, and interacts with the environment to produce your personality (which includes both biological and environmentally-shaped components).
While there is no single agreed-upon model of temperament, researchers believe that your underlying disposition toward sociability, activity level, and emotionality may be strongly tied to your innate temperament.
Many experts believe that people with borderline personality disorder (BPD) are born with a disposition toward very strong emotional reactions. This may be one of the risk factors for the disorder.