The boot process has been changed over a period of time. As I understand it, boot process has been broken into two pieces, one is the "Boot Manager", which finds, presents and controls what can be booted, and the other is the "Boot Loader", which actually does the booting (load and run) of whatever is selected by the Boot Manager.
When the computer is initially powered on, the Boot Manager looks around and decides what candidates for booting are available, and which one has first priority. Unless it is interrupted (by you pressing the boot selection key) it will then try to pass control to whatever boot loader it decides comes first.
I hope it would be clear now :)