journalctl is a command-line utility in Linux systems that allows users to query and view logs collected by systemd's logging service, known as the journal. This logging service captures a wide range of system events, including kernel messages, service status changes, user logins, and more, providing a complete view of system activity. Users can use journalctl to filter logs based on various standards such as time range, severity level, specific units (system services), or even custom fields.