The term boot describes the process the computer performs when it is turned on, loading the operating system and preparing the system for use.
Animation of computer boot process
Boot, boot up, and start up are synonyms and generally describe the long list of things that happen from pressing the power button to a fully loaded and ready to use session of an operating system, such as Windows.
When you turn on your computer using the power button, the power supply provides power to the motherboard and its components so that they can perform their role in the overall system.
The next step is controlled by BIOS or UEFI and starts after POST. This is when POST errors are given if there is a problem with any of the hardware.
