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Both policers (aka rate-limiters) and shapers allow you to restrict bandwidth utilization to less than physical capacity. For example, if you had 100 Mbps ingess port sending traffic to a 10 Mbps port, you might use a policer or shaper to "emulate" similar traffic forwarding behavior on a router with both 100 Mbps ingess and egress ports.
Assuming we want to emulate 10 Mbps egress on 100 Mbps, a 10 Mbps policer's traffic forwarding would behave much like a physical 10 Mbps interface with a shallow egress queue. I.e. too much traffic in too short an interval excess traffic gets dropped.
A 10 Mbps shaper's traffic forwarding would behave much like a physical 10 Mbps interface with a much deeper egress queue. I.e. too much traffic in too short an interval excess traffic gets queued.
Burst interval or burst sizes are somewhat like changing an interface's queue/buffer resources for drop management.
Policers, since they don't actually queue, can be used for ingress or egress. Policers can also be user to mark rather than drop.
