Apache Derby is an open-source relational database management system (RDBMS) that is written in Java. It is designed for easy integration into Java applications and can run in embedded mode or as a server. Derby supports standard SQL and provides a lightweight, reliable solution for developers needing a database without the overhead of larger systems. One of Derby's key features is its small footprint, making it suitable for applications with limited resources. It also supports transactions, indexing, and stored procedures, allowing for efficient data management. As part of the Apache Software Foundation, Derby benefits from community support and regular updates.