What is a relational database?
The information contained in any one table might in and of itself tell us very little. The Contact Types table, for example, provides relatively little information that stands on its own: family, friends, relatives, companies, recruiters, and network.
However, if you could relate this table to the Contacts table—name, address, and phone—you could separate contacts by category. The Contact Types table becomes useful, therefore, when it's related to another table.
Similarly, by relating the Calls table with the Contacts table you can relate specific phone calls to specific people.
The power of a relational database is twofold: Information is managed in separate tables to make maintenance easier, and data can be combined by relating different tables.