A relational database has more than one table and the tables are linked using key fields. For example, a library database could have three tables:
Customer - when a customer joins the library a record is created. It stores their details such as their first name and surname and includes a unique Customer ID.
Book - each book in the library has a record. It stores details about the book, such as the author and title and includes a unique book ID.
Lending - when a customer borrows a book, the lending table stores the customer's unique ID and the book's unique ID in a record. The record could also include additional information such as when the book was borrowed and when it's due back.
The customer and book ID are both examples of key fields.
The book's details and the customer's details need only be entered into the database once.
Because of this, mistakes are less likely to happen and if there were a mistake in a customer's record, for example, correcting it will correct the mistake database-wide.
Duplication is avoided - this keeps the database's file size down.
Details about books and customers are easily accessible using their unique IDs.
Queries can be performed and reports generated, eg a list of books a customer has borrowed since joining the library.