A partition is the process of logically dividing a hard disk into separate volumes. A partition can be used to install separate operating systems or just to store your personal data on a different logical drive.
Each partition has it own label, such as, the partition which has Windows operating system installed in it, is labeled as «C:» by default. Custom labels can be assigned to partitions, if they are not already in use. The labels are used to uniquely identify each volume or partition.
In some cases, a single HDD may contain multiple operating systems. The information of partitions in which the operating systems are installed, is stored in the first sector of the HDD. This sector contains all sort of information about the partition, such as, their address, size, and some other information required to load the operating system.
Since a HDD may contain multiple OS, it gives you the option of choosing which OS to boot, when the computer is powered on. This program is called as a boot loader, which contains record of the operating system installed in the HDD. Once the operating system is selected, it will load the operating system into the random access memory in order to successfully boot the operating system.