Depression, manic depression, and bipolar disorder are all related depressive disorders.
Depression, clinically known as major depressive disorder (MDD) is a pattern of persistent feelings of hopelessness, sadness, and lack of motivation. It may also be accompanied by feelings of guilt, anger, worthlessness, and thoughts of dying or suicide.
Manic depression is a type of depression in which the person is on the opposite end of the spectrum. During an episode they feel elated, like everything is the best it can ever be, and they are sometimes impulsive, act without thinking, and can (in some cases) be dangerous to themselves. During a manic episode the person may spend a large amount of money (including maxing out credit cards, taking out large loans, and making extravagant purchases), take part in activities they might not normally do, such as cheating on their romantic partners, changing jobs, or being promiscuous. After a manic episode the person often feels depressed, ashamed, and embarrassed about what they did.
Bipolar disorder (BD) is a combination or major depressive disorder and manic depression. People with bipolar regularly cycle through both states of depression over the course of a few weeks, months, or years. Bipolar disorder is characterized by this changing of depressive states: depressed, manic, depressed, manic.
Bipolar disorder is often incorrectly though of as rapidly-changing emotional states, such as within hours or days. Typically bipolar disorder states change over a few weeks or months.
If you or someone you know is struggling with depression or a depressive disorder, talk to a licensed mental health care professional or a doctor to receive an assessment, diagnosis, and treatment plan.