Traffic policers and shapers are mechanisms of quality of service that classify and condition traffic.
Policers will drop or re-mark incoming or outgoing traffic that goes beyond a desired rate.
Shapers buffer and delay egress traffic rates that peak above a desired rate until the egress traffic drops below that desired rate. If it below the desired rate traffic is not queued.
To summarise, policers will drop excess traffic and shapers will buffer and delay excess traffic.