"Custody" in ordinary sense, means control or charge over another person. It implies physical detention of a person from movement based on free will. 'Custody' and 'arrest' are not synonymous. A person may be in custody yet not arrested.
"Arrest" is the forceful confinement of a person by police. A person is said to be under arrest when his movements are not free but restricted by police. Section 46 Cr.P.C. explains the procedure of arrest.
When a police officer arrests a person on the suspicion of having committed a cognizable offence, the arrested person is said to be under police custody. The purpose of police custody is to interrogate the suspect to gather more information about the crime, and to prevent destruction of evidences and intimidation of witnesses. According to section 57 Cr.P.C. this custody cannot exceed 24 hours without the orders of a Magistrate.
Every person who is arrested and detained in custody by police should be produced before the nearest Magistrate within a period of 24 hours of such arrest, excluding the time necessary for the journey from the place of arrest to the court of the Magistrate(Article 22(1) of the Constitution of India.)
When an accused is under direct control or custody of the Court, whether Magistrate or Sessions Court or High Court, he is said to be in judicial custody. The accused sent to jail is in judicial custody.
In police custody, the police will have physical custody of the accused. So, when remanded to police custody, the accused will be detained in the lock-up in the police station. In that scenario, the police will be having all time access to the accused for interrogation.