Operating System Boot Process.

Hard Disk and Partitions:-

Partitioning is a process of dividing the Hard disk into several chunks, and use any one of the partition to install OS or use two or more partitions to install multiple OSes. But you can always have one partition, and use up the entire Hard disk space to install a single OS, but this will become data management nightmare for users of large Hard disks.

This is where advantage of partitioning lie!

Now, because of the structure of the Master Boot Record (MBR), you can have only four partitions, and these four partitions are called Primary Partitions.
Again, If we have a large hard disk, we can not have only four primary partitions, hence Extended Partition is introduced. This Extended Partition is not a usable partition by itself, but it’s like a “container” and it is used to hold Logical Drives!
That is this Extended Partition can be subdivided into multiple logical partitions.

In order to boot into a Partition, it must be designated as bootable partition or Active Partition. Active Partition is that partition which is flagged as bootable or which contains OS, this is generally a Primary Partition.