Can you explain what is meant by the term 'key' in the context of data storage or database management systems?

In the context of data storage and database management systems (DBMS), the term "key" usually refers to a unique identifier of a record or a set of data. A key is used for quick search, sorting, and ensuring data integrity.

Main types of keys:

• Primary Key — a unique identifier for a record in a table, which cannot be NULL and must be unique.
• Foreign Key — a field or set of fields that references the primary key of another table, establishing a relationship between tables.
• Unique Key — guarantees the uniqueness of values in a column but can allow NULL.

Example: in a users table, the user_id field can be a primary key, uniquely identifying each user.