bandwidth command is for CBWFQ configuration so for congestion management so used only when there is congestion
whereas police command is for traffic policing that is to limit traffic inbound or outbound on an interface and drop packets which are above the configured rate and so it is used when there is no congestion.
I think you mean, "What is the difference between the bandwidth command and plice command when used in a policy-map."
You use the police command to specify a maximum rate for a traffic class. If the rate is exceeded, an immediate action must be taken, such as drop or remark. Traffic "policing" propagates bursts, resulting in a saw-tooth appearance in the traffic output. In contrast, traffic "shaping" retains the excess bursts in queues for later transmission, resulting in a smoothed appearance in the traffic output. Policing can be applied inbound or outbound, while shaping can be applied outbound only. Both the police and shape commands restrict the maximum output rate, neither of them provides a minimum bandwidth guarantee during periods of congestion.
For periods of congestion you must use either the bandwidth command or the priority command. These commands guarantee a minimum bandwidth to a traffic class during periods of congestion.
Below is a nice PDF by Cisco. Give it a read, I believe it will help.