After a system is shut down, it is booted by using the boot command at the PROM level on a SPARC based system or by using the boot command at the Primary Boot Subsystem Menu on an Intel system.
A system can be rebooted by turning the power off and then back on. This is not a clean shutdown because system services and processes are terminated abruptly. However, turning a system's power off and back is an alternative for emergency situations.
SPARC based and IA based systems use different hardware components for booting. These differences are described in Chapter 12, The Boot Process (Reference).