a) MBR

b) GPT

MBR refers to master boot record and is knowledge of the first sector of any hard disk that identifies how and where an operating system is located so that it can be boot (loaded) into the computer’s main storage or random access memory.

GPT refers to GUID Partition Table  is a standard for the layout of the partition table on a physical storage device used in a desktop or server PC, such as a hard disk drive or solid-state drive, using globally unique identifiers (GUID).

GPTs use logical blocking address (LBA) in place of the historical cylinder-head-sector (CHS) addressing. In a GPT, the first sector of the disk is reserved for a “protective MBR” such that booting a BIOS-based computer from a GPT disk is supported, but the bootloader and operating system must both be GPT-aware. Regardless of the sector size, the GPT header begins on the second logical block of the device