Hi BethRags. I'm not a mental health professional, but from all of my research the following are the core similarities/differences:

• Bipolar type 2 - Experienced(s) a full depressive episode(s) that meets the criteria of the DSM, AND, experienced(s) at least one hypomanic episode (never full manic) that meets the DSM criteria. Hypomania is not as severe as full blown mania, and does not result in psychiatric hospitalization. One can be hospitalized for bipolar 2 depression or mixed states, but the mixed state's hypomanic portion is not as severe as full blown mania. It is generally the case that people with bipolar 2 experience much more depression than hypomania. One study projected that the average period spent ratio is 39:1 (depression to hypomania).*

• Bipolar type 1 - Experienced(s) at least one full blown manic episode that meets full DSM criteria and is more severe than hypomania. The period must be a minimum of 1 week, symptoms most of the day, OR severe enough to warrant psychiatric hospitalization. Though most people with bipolar type 1 do experience depressive episodes, depression is not required for a bipolar type 1 diagnosis.The study mentioned above stated an average of 3:1 (period in depression compared to mania).* People with bipolar type 1 can experience hypomania, but again, there must be a full blown mania for this diagnosis.

• Schizoaffective Disorder -Update - I've removed my bit about this because of lack of full understanding on my part.

I have read many people ask the question about how you know when an elevated episode is hypomanic vs. manic.Saying "full blown mania is more severe" is the case, but as for the dividing line, generally only a psychiatrist or psychologist can determine the difference. As stated in the bipolar type 1 bullet, only full blown mania (not hypomania) generally results in psychiatric hospitalization. That doesn't mean everyone with full blown mania ends up in the hospital. Psychosis can be a clear indicator of the difference. If no psychosis is present, I guess the severity of the damage and dysfunction in life is considered. In both cases of hypomania and mania, the behavior is noticeably different than baseline/stable behavior, but the dysfunction is noticeably different between hypomania and mania.

Resources:

1. DSM-5 (Diagnostic and Statistical Manual of Mental Disorders - Fifth Edition)

2. Statistics on average ratios of depression to hypomania (or mania) appears in several articles online, including The National Center for Biotechnology Information journal article at Treatment of acute bipolar depression which references the following:

Judd LL, Akiskal HS, Schettler PJ, Coryell W, Endicott J, Maser JD, et al. A prospective investigation of the natural :history of the long-term weekly symptomatic status of bipolar II disorder. Arch Gen Psychiatry. 2003;60:261–9.