Have you ever given it a thought what happens when you double click on an icon of application or some file. During the course of time this operation is performed, you might see flickering of light associated with hard disk and listen to churning sound of hard disk. This definitely means that something is going on inside, rather one might correctly guess that data is being accessed.
To understand this first we need to understand, what is hard disk, how it stores data and how does it access the data.
The hard disks track their origin with IBM 305 computer during the year 1955. The hard disk started as large disks up to 20 inches in diameter holding just a few megabytes of memory. During the starting years they were called "fixed disks" or "Winchesters" (a code name which was generally used for a popular IBM product). They later became known as "hard disks" so as to distinguish them from "floppy disks." As the 1980s began, hard disk drives were a rare and very expensive and were very rare on personal computers (PCs); however by the late '80s, hard disk drives were standard on all Personal Computers.
A hard disk contains a set of electromagnetic platters (consider it as a plate with fixed size hole at centre of them or CDs) stacked on top of one another (a bit like many CDs in a stack) with a narrow gap between each platter. These platters typically spin at 3,600 or 7,200 rpm when the disk is operating. Each platter is usually double-sided i.e. each platter has 2 sides to store data, and each side has its own read/write head. So, if you've got 4 platters, you've probably got 8 heads.
Each platter has set of concentric rings (technically called a "track") which are used to store the data and each head reads from one of these concentric rings on the cylinder. There can be more than a thousand tracks on a 3.5-inch hard disk. All the heads move at the same time and are positioned to read or write to the same track on their respective platter, which means that they form a cylindrical shape and hence are known as cylinder. So, if head 2 is positioned to read from track 23, head 3 will also be positioned to read from track 23. Therefore, we might say that head 2 is positioned to read from cylinder 23 (which implies that head 3 and further heads are also positioned to read from same track i.e. cylinder 23).
Finally, each track is split into small segments. Each segment is called a "sector", as shown in the figure. A sector is the smallest physical storage unit on a disk, and is most of the time 512 bytes (0.5 kB) in size. All the hardware operations take place in terms of sectors.
If some application or some file wants to access one particular sector, then it could refer it by specifying which head it is on, and which cylinder it is on and finally the appropriate corresponding sector. That would then uniquely identify the sector that we wanted to access.