Grub A Popular Linux Boot Loader (*1)
Windows Boot Manager
Macintosh BootX
You May not see the Boot Loader if you have a single Operating System on your computer
There are actually several Linux Boot Loaders including:
GNU Grub, rEFInd, Clover, LILO, BURG, Syslinux, Gummiboot, Kboot, Gujin, Das U-boot