What are the critical differences between bipolar disorder and schizophrenia?

What makes them different from each other? I know that they share some symptoms in common, like hearing voices and hallucinations.

Schizophrenia and bipolar disorder are are often misdiagnosed as one another, and this is not so surprising when we consider how much the two disorders have in common. For example:

Schizophrenia and bipolar disorder share medications. Five of the current atypical antipsychotics originally approved to treat schizophrenia are now also approved as treatment for acute mania. Two of them are even approved as maintenance treatments for bipolar disorder. The fact that these two diseases are helped by some of the same pharmacological treatments indicates that they may have similar pathophysiology causing the symptoms.
The positive symptoms of schizophrenia can look like the symptoms in about 50% of manic episodes, epecially those with psychotic features. (These can include delusions of grandeur, hallucinations, disorganized speech, paranoia, etc).
The negative symptoms of schizophrenia can closely resemble the symptoms of a depressive episode (these include apathy, extreme emotional withdrawal, lack of affect, low energy, social isolation, etc).
The two disorders share abnormalities in some of the same neurotransmitter systems. For example, both depressive episode symptoms and the negative symptoms of schizophrenia are at least partially mediated by serotonin. Likewise, the positive symptoms of schizphophrenia and the symptoms of mania are mediated in some way by excesses of dopamine signalling. The atypical antipsychotics approved for both these disorders work on both the serotonin and the dopamine systems.
However, despite a significant overlap in symptoms, treatment, and pathophysiology, the current system for diagnosing psychiatric disorders in Western medicine (in the U.S., the DSM-IV contains all the guidelines for psychiatric diagnosis) emphasizes a categorical separation between diseases. In other words, it emphasizes differences, not similarities. In the past this differential diagnosis separation was more important; before the advent of the atypical antipsychotics, first-generation neuroleptics used for schizophrenia actually made bipolar disorder worse.

bi-polar disorder is when you have super high highs and super low lows randomly. severe mood changes. schizophrenia is when you hear voices and have hallucinations... there is like two sides of you... like your two differnt people. watch the movie secret window with johnny depp! <3
Actually victims of bipolar disorder do not hear voices, that is a trait unique to schizophrenia, and schizophrenia alone.
Bipolar dissorder - Maniac/depresive
The states switch and alternate between two abnormal points.

Have a look at this links:

http://en.wikipedia.org/wiki/bipolar_dis...

http://en.wikipedia.org/wiki/schizofreni...
You know what I'm not sure really but if you find out please let me know. The reason that I say this is a Dr. diagnosed my mom with Schizophrenia and now her new Dr. is saying that it is bipolar but the medicine that they are proscribing her doesn't work.
People are more nuts with schizophrenia and cant usually work
with bipolar it can be medicated and they can work but never make up their minds well.
Well, interesting question. Bi-polar disorder is commonly associated with unstable changes in the serotonin and melatonin levels in the brain causing severe "mood swings". Bi-polar can cause aural and sensory hallucinations (paranoia etc.) as well as extreme aggressiveness, panic attacks, absentee seizures, Schizophrenia is attributed to a "miss-fire" in the brain's wiring. It can cause severe hallucinations and mental incapacity with the MANY traumas associated with the assault on the brain.
Hope that helps a bit...Cheers.