Final answer:
A candidate key is an attribute or a set of attributes that can uniquely identify any tuple in a table and can serve as the primary key. A table can have multiple candidate keys but only one primary key, which is used to uniquely identify tuples. Unique keys and foreign keys serve different purposes in a database.
Step-by-step explanation:
An attribute that can uniquely identify any tuple without referring to any other data is described as a candidate key. A relation/table can indeed have multiple candidate keys, which are essentially potential choices for the primary key, but only one can be selected as the primary key. The primary key is a special candidate key that has been chosen to uniquely identify tuples in the table. The candidate key is an attribute or a set of attributes that could act as a primary key based on its uniqueness and non-null constraint. A foreign key is a key used to link two tables together, which distinctively differs in purpose from a candidate or primary key. A unique key, on the other hand, also ensures uniqueness of tuples but does not necessarily have to be chosen as the primary key.