First of all, the answers listed on this thread are only PART of the complete answer. I have provided, in the source section, a link to the website that deals with bipolar disorder.
This diease WAS FORMALLY known as manic depressive disorder, however, as not all bipolar people exhibit the signs of manic depressiviness, the 2 disorders were eventually seperated and made distinct to one and other.
Bipolar is simply a form of a depressive state that can make a person have dramatic mood swings over a long or short period of time. Most people that suffer from bipolar disorder can be just fine for many weeks or months, and then all of a sudden turn to a major depressive state, be self destructive, or perhaps even suicidal in nature; and then a few more weeks or months down the road, they will be just fine again.
This bipolar disorder has to do with a chemeical imbalance in the brain between seritonin and other chemicals that inihibit our natural ability to control impulses and deprssive states.
This is however a condition that responds well to medication and therapy. So, please, dont be worried too much about them. They will be just fine, your job would be to be supportive, and be their friends, no matter what.
Please see the website below for more information on bipolar disorder.