Ubuntu - Boot - Master Boot Record (MBR)

The Master Boot Record (MBR) resides in the first 512 bytes of the hard disk. This 512 bytes contains two things:

440 bytes for the boot loader (Grub, or Windows boot loader, etc.)
The remaining space is used for the partition table, i.e.: there can only be 4 primary partition in an MBR partitioning scheme. This remaining area also has the partition location information.

This can be done by two methods:

CHS method: In this method a partition is located through cylinder, head, sector. This is described by a 24 bit number, and in this case it can only mention disk to a maximum of 8GB.
LBA method: LBA stands for Logical Block Addressing, where each and every sector is numbered serially resulting in easy addressing of spaces upto 2TB.

Cons of the MBR partitioning scheme

It only allows four primary partitions.
The workaround for creating more than four partitions by making one of them extended (containing logical partitions inside that) also leads to problems sometimes as some operating systems can only boot from Primary partitions.
The CHS implementation of the MBR partition table can lead to problems because the Cylinder, Head, Sector geometry of the disk can change.
The LBA scheme has a 2TB limitation.
Logical partitions are stored in a Linked List data structure over the extended partition, and can easily fail, which internally can make some logical partitions inaccessible.