An arrest is when a person is physically detained by the police. No person can be detained by the police without being informed of the reasons and the basis in law for their arrest.
Generally, the police need a warrant to arrest someone. Crimes for which a warrant is required are called non-cognizable offences. On receiving information of a non-cognizable crime, the police must take permission from the Magistrate to arrest someone. This permission from the Magistrate is known as a warrant.1