There is no difference between bipolar disorder and manic depression. Bipolar is the modern term for manic depression. The term "manic depression" is much misunderstood. Some people think "manic" means "really depressed." Mania is the 'opposite' of depression. A manic depressive is someone who cycles between being manic and depressed. The term "bipolar" supposedly makes this more clear.
The words manic depressive were previously used in medical literature, but now that we have come out with new disorders we thought we'd rename the previously benign-sounding manic depressive, who had worth and human dignity and was capable of creative endeavors the more hyped up current terminology, which I personally believe only serves to further undermine the success of treatment by further stigmatizing people who are considered strange or hard to understand. In addition to changing the name, they have added new categories in the dsm-v (the bible of craziness) and made up new symptoms (they didn't have shopping freaks in the 1800's) to reflect current socio-cultural conditions.