Important Database Concepts - Microsoft Business Intelligence Tools for Excel Analysts - page 14

With the key fields added to each table, you can now find a field in each table that links it to other
tables in the database. For example, Table 1-4 shows CustomerID in both the Customer table (where
it's the primary key) and the Invoice table (where it's a foreign key).
This way, you're not repeating information on every row, you're just providing a link back to the table
containing the information that may be required to show up on the report or invoice. The database
will handle retrieving all related information so you don't have to worry about it — the only thing you
have to define is the link between the tables.