OwnCloud is a free and open-source file sharing and storage system that allows users to host their own cloud server on a Raspberry Pi. It provides features like file sharing, remote file storage, data synchronization, and access to documents across devices. The system stores files in a user's own directory structure and includes additional apps for tasks, calendar, notes, and user management. Users have full control over their data and can choose OwnCloud as an alternative to commercial cloud storage services.