What is known as Police bail is granted after arrest but BEFORE charging. The time between bailing and the defendant returning to the police station to answer his police bail is used to further investigate/find evidence. When the defendant answers their police bail a decision is then made on the strength of the evidence whether to charge/report or take no further action.
On Bail means they have been arrested and charged and are awaiting a court appearance or they have appeared in court and the court has granted bail for the defendant to appear at a later date