The arresting officer is the policeman or woman that reads a suspect their Miranda rights when they are being taken to jail. The arresting officer is usually also the officer that takes the person into custody by putting handcuffs on them.
The arresting officer is responsible for filing paperwork with the jail that is then sent to the court. This paperwork outlines the reason why they arrested the suspect and notes the evidence that was present against the person in relation to the crime.