both are mood disorder. mood can be 2 ways, up and down. when one get too high, it's call manic, while when the mood goes too down, it's call depression.
when a person has bipolar, means his mood can swing up and down.
while for a depression patient, his symptoms is always down.

it's quite easy to differentiate bipolar from depression if a good history is taken from relatives and patient. in Bipolar patient, they often has history of elevated mood, fights of idea, spending spree, reduce sleep, (there's a criteria to diagnose this disease) while for depression, they must also meet a sets of criteria.(low mood, loss of interest, hopeless, guilt, want to die, loss of appetite, weight, insomnia/sleep a lot)