Derby is one of many relational databases available to you. Others are Oracle, SQL Server, MySQL, PostgreSQL, and Hypersonic SQL.
Some are open source (MySQL, PostgreSQL, Hypersonic SQL, SQL Lite); others require a license (Oracle, SQL Server).
Some require installation (MySQL, PostgreSQL, Oracle, SQL Server); others do not (SQL Lite, Derby, Hypersonic SQL).
Java interacts with all databases using JDBC. You get a JAR of classes from the vendor that implement the interfaces in the java.sql
package for that particular database.