Preventive detention is an imprisonment that is not imposed as the punishment for a crime, but in order to prevent a person from committing a crime, if that person is deemed likely to commit a crime. In most democracies, no one can be arrested without being told the grounds for such an arrest, except under rare and special circumstances (usually anti-terrorism legislation). An arrested citizen has certain rights: He/she must be subject to and informed of a criminal charge and brought before the nearest magistrate within a certain amount of time, and has the right to defend himself by a lawyer of his choice.
Depending on the laws, this lawyer can be called for as soon as the detention starts, or sometimes days or weeks later. In contrast to this, under preventive detention the government can imprison a person for some time without a criminal charge. It means that if the government feels that a person being at liberty can be a threat to the law and order or the unity and integrity of the nation, it can detain or arrest that person to prevent him from doing this possible harm. Some jurisdictions allow preventive detention only in specific cases, for example only for persons who have already been sentenced for a serious crime.
A related, but different form of detention is detention of suspects. In contrast to preventive detention, detention of supects must quickly be followed by a criminal charge (or happen after the charge). In most jurisdictions, people suffering from serious mental illness may be subject to involuntary commitment under mental health legislation. This is undertaken on health grounds or in order to protect the person or others. It does not strictly speaking constitute a form of preventive detention, because the person is detained for treatment and released once this has proved effective.