As someone who has been diagnosed with all three at different times...

Bipolar 2: Severe depression with mild mania that doesn't interfere with life
Bipolar 1: Severe depression AND mania that interferes with life and/or needs hospital to keep safe
Schizoaffective: Same as the bipolars, but with the addition of psychotic symptoms (hallucinations and delusions) that occur even when mood is normal ("euthymic")