Procedural memory is the long-term memory of skills and procedures, or "how to" knowledge.
As compared with declarative memory, it is governed by different mechanisms and different brain circuits. Procedural memory is often not easily verbalized, but can be used without consciously thinking about it; declarative memory can be put into words. Examples of procedural learning are learning to ride a bike, learning to touch-type, learning to play a musical instrument or learning to swim. There is no simple stimulus-response pairing. Instead, the brain tries to figure out optimum memory pattern by trial and error. Procedural memory can be very durable.
Studies of people with certain brain injuries (such as damage to the hippocampus) suggest that procedural memory and episodic memory use different parts of the brain, and can work independently. For example, some patients are repeatedly trained in a task and remember previous training, but don't improve in a task (functioning declarative memory, damaged procedural memory.) Other patients put through the same training can't recall having been through the experiment, but their performance in the task improves over time (functioning procedural memory, damaged declarative memory).
Damage to the cerebellum and the basal ganglia seems to particularly affect procedural learning.