The main boot record, also known as master boot record or MBR, was introduced alongside disk partitioning for the DOS 2.0 operating system in 1983. As such, it's seen exclusively in storage media that is partitioned – including hard disk drives (HDDs), solid-state drives, and more. It's not necessary for an un-partitioned device or disk, so it's not found in floppy disks or similar media.

What's the Purpose of the MBR?
It's the MBR's job to store information related to the storage medium's partitions and file system. Additionally, it includes executable code – sometimes referred to as a boot loader – that loads the critical files needed for the operating system (OS).

• Partition Table: A simple table that describes all of the virtual partitions within a logical drive or other storage medium.
• Bootstrap Code: This data tells the system how to identify the bootable partition. It also tells the system how to load and execute the volume boot record (VBR).
• Disk Timestamp: When you format a drive and create a new file system on a partition, a disk timestamp is generally made to mark the current date.
• Disk Signature: This is a unique identifier that helps your OS distinguish the drive from other drives and storage mediums.

Since the data contained within is so vital to the system's day-to-day functionality, it's critical that your MBR stays intact and free from data loss or corruption. Because of its importance, the MBR also plays a large role in many data recovery operations.

Common Sector Layout
The MBR itself is not located within any partition at all. Instead, it exists within the first sector of a drive or device -before the first partition even begins. There are exceptions to this rule, but they usually occur when using specialized boot managers in a multi-boot system.

Likewise, there are four primary partition table entries located in the MBR, with each one taking up 16 bytes of space. The bootstrap code area, which is located before the individual partition table entries, comprises 446 bytes, while the boot signature, which directly follows the four partition table entries, takes up 2 bytes. In total, the classic MBR structure requires 512 bytes of space.

However, this varies greatly when using later versions of DOS or when using specialized disk management software. Some of these systems feature five, eight, or 16 primary partition table entries.

Limitations of the MBR
But the MBR is limited to partitioned disks of 2 TiB (232 × 512 bytes) or less. Note that this uses a unit of measurement known as a tebibyte. Unlike the terabyte, which is based on the power of 10, the tebibyte uses a binary system that is based on the power of two. Essentially, this means that the classic MBR has a maximum capacity of 2.2 TB per drive or device. This restriction has many modern computer users favoring the GUID Partition Table, or GPT, over the classic MBR structure.

You may read more about Master boot record in Wikipedia: Master boot record.