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The priority command sends traffic to a single queue, the low latency queue (LLQ). You can have more than one class defined with priority, but what that does is rate limit each class independently as the traffic is queued into the single LLQ. As the other posters have noted, the implicit policer only engages if there's congestion. It also drops traffic, when it's active, that exceeds its rate limit.
A police statement is a rate limiter added to a policy map class; even a priority class. Unlike a priority class's implicit policer, it's always active. It's more flexible than a priority class's implicit policer as you can adjust Bc and Be, often configure average or peak measurement, and define different actions for traffic that's within rate, overrate and very much overrate (features available may vary per IOS/platform). (Note: a policer doesn't have to always discard overrate traffic, it might just mark it differently.)
Also, priority is an egress only policy feature. A policer can be use on ingress or egress policy.
