Surrogate Key
A warehouse gives each dimension row its own meaningless number, so one customer can have several historical versions, each with a key a fact can point to.
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Durable Key
When an employee rejoins with a new number or two customer records turn out to be one person, a durable key keeps a single identity across every change.
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Natural Key
A customer number or SKU identifies a record in its source system. It is essential for tracing data back, and unreliable as the only key a warehouse joins on.