Surrogate Key:
Surrogate keys are keys that have no “business” meaning and are solely used to find a record in the table. Such keys are either database generated (example: Identity in SQL Server, Sequence in Oracle, Sequence/Identity in DB2 UDB etc.)
Natural Key:
Keys are natural if the attribute it represents is used for identification independently of the database schema. What this basically means is that the keys are natural if people use them example: Invoice-Numbers, Tax-Ids, SSN etc.