HDD stands for Hard Disk Drive. The history of HDD goes way back. HDDs were introduced by IBM in 1956. HDDs have storage disks also known as platter that rotates rapidly. A mechanical arm reads and writes the data into the platter. There is a lot of mechanical process going on which makes the HDDs slower than SSDs.
The advantage of HDD over SSD is that it is available in large capacity and is cheaper. For example, WD Blue 1TB HDD costs around 3500Rs and Samsung 850 EVO 250GB SSD costs around 8000Rs. This cost difference is huge. So now let us compare the key factors for the SSD vs HDD