Novice Question: How can you have a relational database at all without foreign keys?

In other words, if your tables aren't referencing other tables, how do you perform JOINS for your queries? From my limited experience, they seem pretty essential for most databases. Or is the article just talking about foreign key CONSTRAINTS and cascading UPDATES and DELETES, rather than literally talking about the values stored in foreign key columns?

If anyone can explain, I would greatly appreciate it.