Java DB is an open-source relational database management system based on the Apache Derby project. It is written in Java and designed to be lightweight, making it suitable for both embedded and server-based applications. Java DB supports standard SQL and provides features like transactions, security, and multi-user access. As part of the Java platform, Java DB is often used in conjunction with Java applications, allowing developers to easily integrate database functionality. It is particularly useful for small to medium-sized applications due to its simplicity and ease of deployment.