What is HDD? Difference between HDD and SDD?

The hard drive stores all your data in a disk. It stores the data in the hard disk, where all your files and folders are physically located to a hard disk. A hard drive is only slightly larger than your hand, yet it can hold over 100 GB of data. The data is stored on a stack of disks that are mounted inside a solid encasement. These disks spin extremely fast (typically at either 5400 or 7200 RPM) so that data can be accessed immediately from anywhere on the drive. The data is stored on the hard drive magnetically, so it stays on the drive even after the power supply is turned off.

The information is recorded in bands: each band of information is called a track.

The term "hard disk" refers to the actual disks inside the drive. However, all three of these terms are usually seen as referring to the same thing -- the place where your data is stored. Since I use the term "hard drive" most often, that is the correct one to use. Each platter has the same number of tracks and a track location that cuts all across all platters is called a cylinder. The tracks are divided into pie-shaped sections known as sectors.

A computer needs an operating system to allow users to interact with operating system and perform tasks in it. The operating system interprets keyboard and mouse movements and allows for the use of software and hardware like a browsing, word processing, playing video games and listening to music ♬ To install operating system you must need a computer system, a hard drive (or another storage device) is required. The storage device provides the storage medium where the operating system is installed and stored.

A hard drive is also required for the installation of any programs or other files you want to keep on your computer. When downloading files to your computer, they are permanently stored on your hard drive or another storage medium until they are moved or uninstalled.

Data sent to hard drive and read from the hard drive is controlled by the disk controller available in a computer system. This device tells the hard drive what to do and how to move its components according to user need. When the operating system needs to read/write information in a hard disk, it examines the hard drive's FAT (File Allocation Table)  to determine file location and available write areas. Once that is determined, the disk controller instructs the actuator to move the read/write arm and align the read/write head. Because files are often scattered throughout the platter, the head needs to move to different locations to access all information.

All information saved and stored on the hard drive in a computer, The data is stored in hard disk magnetically. If the computer needs to read information from the hard drive, 

it would read the magnetic polarities on the platter. One side of the magnetic polarity is 0, and the other side is 1. Reading this as binary data, the computer can understand what the data is on the platter.

For the computer to write information to the platter, the read/write head aligns the magnetic polarities, 

writing 0's and 1's that can be read later.

In a computer system there is a hard drive that stores all the software installed on a computer, as well as all the data files or folders created by user used by this software. In a Hard drive it includes all the documents that you have created or downloaded from Internet, like photos, music, video etc. The  Computer Hard drive is a type of permanent storage, rather than a temporary memory like your RAM (Random-Access-Memory). RAM is a type of Volatile memory which means it loses its data when you turned off the computer. Hard drive means that when you finished your work and turning off the computer the files remain safely stored on the Hard drive so you can use them again in the next time when you start your computer. It doesn't loses its data when it is switch off.

1. Hard disk drives(HDD) :-  Hard drive uses one or more rotating discs and rely on magnetic storage. Hard disk is a non-volatile storage and Random access digital data storage device.

2. Solid-state drives(SSD) has no moving mechanical parts, but Solid-state-drive uses flash memory such as found in USB(Universal Serial Bus) flash drives. If you work with a regular desktop computer you should go for hard disk drive(HDD). Solid-state drives are for high-end uses, and it is used in expensive laptops. In SSD files load faster than HDD. It is more reliable and faster than Hard-disk-drive.

In every computer system at least one internal Hard drive to store software programs and data files to the computer. If you are using a Windows Operating system, most of the user have by default their dive named C Drive where you can store all the data files and programs installed on your computer. In case you are a Mac user, in Mac it is called Hard Drive. If you need extra storage capacity in your computer system you need to install additional Internal Hard drives or connect separate external Hard drives to store more files..

Most basic hard drives consist of a number of disk platters that are positioned around a spindle inside a sealed chamber. The chamber also includes read-and-write heads and motors. 

The motor is used to spin the platters, which hold the data, at up to 15,000 RPM. As the platters spin, a second motor controls the position of the read-and-write heads that magnetically record information to, and read information from, tracks on each platter.