On Linux systems, run levels are operational levels that describe the state of the system with respect to what services are available.
One run level is restrictive and used only for maintenance; network connections will not be operational, but admins can log in through a console connection.
Others allow anyone to log in and work, but maybe with some differences in the available services. This post examines how run levels are configured and how you can change the run level interactively or modify what services are available.
Read more at NetworkWorld