What's the difference between bipolar and borderline personality disorder?

Very different. Borderline pd is a personality disorder -- meaning there are fixed behavior patterns that don't change with circumstances, causing significant problems in relationships, work, and school, etc. Bipolar d/o is a mood disorder, and the person could behave normally when not in an episode. Borderline pd may have rapid mood shifts and extreme rage, usually in relation to abandonment issues.
Borderline. Intense fear of abandonment, being manipulative, impulsivity, a tendency to think of people and things in all good or all bad terms, self mutilating behaviors, drug and alcohol abuse, sexual impulsivity, a profoundly disturb sense of self, and extreme emotional volatility. Bipolar disorder is characterized by extreme mood swings from deep depression to the stratosphere of mania.
A lot and a little . This is too big a question to answer in such a small space. What is more interesting is what makes them similar. Bd is a mood disorder with discrete mood episodes and a return to normal between episodes, while bpd is pervasive, characterized by an unstable sense of self, impulsivity and unstable relationships. Still, some think that bpd could be related to bd. We find that meds for bd help bpd.
They may look alike. Bpd is a b cluster personality disorder (dramatic, emotional or erratic) ; includes marked impulsivity ; instability of - mood, interpersonal relationships, ; self image. It can be very dangerous as fluctuations of feelings are extreme ; intense. Bipolar is a mood disorder ; doesn't require the instability - but it's likely to be there. Both are painful ; hard to treat successfully - but worth it.