Bipolar disorder and depression are Mood Disorders.
This means that they are both mental health conditions that affect your mood in a chronic way, it is more than just being sad or happy. Depression is chronic (meaning long lasting), persistent, debilitating sadness/misery/despair. It creates extreme low mood. Bipolar disorder is a mixture of depression and mania. Depression would be the low mood and mania would be the exact opposite which is extreme high mood and happiness which leads to irrational and sometimes destructive behavior.