Wikipedia

• Surrogate key in a database is a unique identifier for either an entity in the modeled world or an object in the database.
• Primary key is a specific choice of a minimal set of attributes (columns) that uniquely specify a tuple (row) in a relation (table).
• Foreign key is a set of attributes in a table that refers to the primary key of another table.
• Persistent object identifier (POID) is a unique identifier of a record on a table, used as the primary key.
• Superkey is a set of attributes that uniquely identifies each tuple of a relation.
• Candidate key of a relational database is a minimal superkey.
• Unique key is a candidate key.
• Natural key is a type of unique key in a database formed of attributes that exist and are used in the external world outside the database (i.e. in the business domain or domain of discourse).
• Composite key is a candidate key that consists of two or more attributes (table columns) that together uniquely identify an entity occurrence (table row).