There are two ways you can be committed to a mental hospital: voluntarily and involuntarily. Voluntary commitment, as you would expect, is when you acknowlege you have a problem that cannot be solved outside of the hospital, so you enter it of your own free will. With this type of commitment you are making your own decision. It is much freer in feeling than the alternative. It also carries much less stigma.
An involuntary commitment occurs when someone else decides that you are a danger to yourself or others and seeks the legal means to have you committed. Each state has its own laws about involuntary commitment, and the process varies quite a bit from one state to another. Involuntary commitment is unpleasant in that you are not in control of the situation. It also carries a heavy social stigma.