The process and events beginning with power being supplied to the processor and ending with the operating system fully loaded and ready for use. Questions about specific parts of the boot process should be tagged with, e.g. [bios].
Computers execute programs and the operating system in working memory (RAM), however operating systems are typically stored on fixed media such as hard disks. Once the computer is turned on, it uses the boot process to find the operating system on fixed media, load it into working memory and then pass control of the computer hardware to it.