GPT vs MBR: what is the difference?
GPT and MBR are two ways of storing a disk's partition table. MBR is older and limited to four main partitions and 2 TB disks; GPT supports far larger disks and many more partitions.
Why it matters when you use a bootable USB drive
UEFI computers normally boot from GPT disks, while older BIOS computers use MBR. A mismatch is a common reason a USB drive will not boot.