I created the following animated GIF for a system administration course that I teach. I found that it's much more efficient to show students how to boot into single-user mode than trying to describe each step of the process using text. (The animation starts at the "vmware" splash screen, ends with executing the "runlevel" command, and then it loops.)
For those who are unfamiliar with runlevels, when Linux boots into single-user mode (or "runlevel 1") no services (e.g., sshd or mysqld) are started, and the user is not prompted for authentication. At this runlevel, typically the only processes running are a few system processes and kernel threads. Obviously, this mode can only be utilized when you have "physical" access to the system (i.e., interacting with a keyboard/monitor that's directly attached).
There are two primary use cases for this mode:
- Maintenance tasks that can't be performed while services are running, such as modifying drive partitions or mount points.
- Nobody knows the administrator's password, including the administrator.