In the 'Orders' table, 'CustomerID' acts as a foreign key because it is a field that references the primary key of another table, 'Customers', to establish a link between records in both tables. A primary key is unique within its own table only and cannot directly act as a key for another table. An index aids in quickly accessing data but does not inherently create a relationship between tables. While 'CustomerID' does uniquely identify each customer, within the context of the 'Orders' table, it serves the specific role of a foreign key.