Bailable warrant is a sort of notice issued by the magistrate, generally to the police to arrest the person concerned named in the notice with the condition that if the said person execute a bond with sufficient sureties (of the amounts mentioned in the warrant) that he will appear before the court on the date and time mentioned in the warrant and thereafter also, then the police officer serving the said warrant shall release the person concerned from his custody on his executing such a bond (Section 71).
Non-bailable warrant (NBW) is also a sort of notice, generally to the police to arrest the person concerned named in the notice but without any condition. The police officer to whom this warrant is addressed is obliged to trace out the person concerned and to arrest him and to produce him before the court without any delay (Section 76).
It is normally issued when the person concerned does not appear before the court despite receipt of summons and/or bailable warrant. The NBW may be cancelled by the court when the person, against whom NBWs have been issued, appear in the court with an application for cancellation of NBWs explaining the reasons for his not appearing earlier. If the court is satisfied, it cancels the NBWs , but if it is not, then it sends him to jail for a few days so that he does not repeat it again.