systemd-logind is a system service that manages user logins. It is responsible for:
• Keeping track of users and sessions, their processes and their idle state
• Creating control groups for user processes
• Providing PolicyKit-based access for users to operations such as system shutdown or sleep
• Implementing a shutdown/sleep inhibition logic for applications
• Handling of power/sleep hardware keys
• Multi-seat management
• Session switch management
• Device access management for users
• Automatic spawning of text logins (gettys) on virtual console activation and user runtime directory management
User sessions are registered in logind via the pam_systemd(8) PAM module.
See logind.conf(5) for information about the configuration of this service.
See Multi-Seat on Linux  for an introduction into basic concepts of logind such as users, sessions and seats.
See the logind D-Bus API Documentation  for information about the APIs systemd-logind provides.
For more information on the inhibition logic see the Inhibitor Lock Developer Documentation  .
Multi-Seat on Linux
logind D-Bus API Documentation
Inhibitor Lock Developer Documentation