How it works
Each table holds one kind of thing (users, orders, products) and every row has the same columns. Rows point at rows in other tables through keys, so an order stores a customer id instead of a copy of the customer, and a query can join the tables back together in one step.
The structure (the schema) is declared up front and enforced by the database, which is what makes relational databases good at keeping data consistent. PostgreSQL, MySQL, SQLite, SQL Server and Oracle are all relational.
Related terms
More in Databases and storage
The two families