just a note on it’s new name: “bi-polar” means “two-poled,” that is, swinging between the two poles of mania and depression. so it’s actually as descriptive as manic-depression, it just hasn’t had time yet to acquire the stigma that prompted the name change.
it’s pretty much the same as it was before, but it’s also true that there are shared factors among the different labels. basically, it’s all human traits taken to an extreme with some chemical imbalances. also, bipolar can become so severe that it closely resembles schizophrenia (hallucinations, delusions), but it’s usually more espisodic. schizophrenia itself isn’t a cut-and-dried thing. there are, for example, “positive” symptoms (hallucinations, etc) and “negative” (flat affect, lack of concentration, etc). it’s not unheard of for doctors to disagree on just what someone has when they’re having psychotic phenomena (diagnosis is an art, not a science, because there’s a subjective element).
also, spot on to the person who said instability is a key trait of borderline. yep, instability in the sense of “emotional lability” meaning, basically, really really really emotional people. the extreme emotionality leads to very unstable relationships, a chaotic job life, and so on. but they’re not psychotic (ie, not hallucinating, they live in the “same reality” as others, just in a more extreme way).
good place to mention this: over the last decade, the idea of borderline as untreatable is changing.