A hard drive is the large memory storage mechanism of a typical computer. It is typically a set of mechanical rotating disks with read/write "heads" that "fly" at sub-micron heights above them, reading and writing data magnetically. The heads typically "seek", moving over the disk's surface to access the desired circular track on the disk. This is in contrast to the RAM, random access memory that the computer accesses on a nanosecond-by-nanosecond basis. The disk drive is typically more than a million times slower than the RAM, but with much larger data capacity. It also has the property that its data persists when the computer is turned off, while RAM often loses its data. When the computer is operating, it requires access to both the RAM and the disk. The operating system performs operations to read data from the disk into the RAM, and proceeds from there.
The computer's programs and files are permanently stored on the disk.
When a computer is "suspended", the operating system stops running and the processor is powered off, but active data is still left in the RAM, so the computer can be restarted nearly instantly. A small amount of power from the battery is required to keep the RAM data from evaporating. When a computer is "hibernated", the active data from the RAM is written so a special place on the disk, and the computer is powered off completely. Saving and restoring the RAM takes several seconds.
The word "hard" refers to a time when such high-performance and high-capacity disks were a novelty, and computers only had floppy disks as their backing storage. These floppy disks typically had a capacity of only a megabyte or so, and the computers were incredibly primitive by today's standards. But the word "hard", and the abbreviation "HDD" lives on.
Even when a computer is configured to start up by network boot (the operating system is retrieved from a server) a hard drive is generally needed.
Disk drives are often replaced with "flash ram" as the technology of the latter improves. Such devices are often called "SSD" for solid-state drive. Flash ram can retain its data when the power is off. Computers using SSD can be "booted up" instantly. The technology of flash ram has not reached the point where it can compete with disks for data capacity, so SSD computers are typically small laptops, whereas computer used for serious things like software development still use disk drives.
There are two types of modern drives, and two kinds of data transfer which can be used by them.
The older but still most common kind of drive is the hard disk drive (HDD). In this kind of drive, a magnetic "platter" is spun by a motor at high speeds past a reader "head." The "head" moves perpendicularly to the disk to read the required information from the disk as it passes by. This kind of drive generally fails based on the time it has been powered and running. This makes it better for high-use systems.
The newer but still less popular kind of hard drive is the solid-state drive (SSD). In this kind of drive, a set of flash memory is installed in a fixed position, and read using electrical impulses send to each bit location. All information can be accessed without needing to collect it from various locations on a disk drive, and since this is not limited by the rotational speed, it can preform more quickly. This kind of drive generally fails based on the amount of data changed on this drive. This makes it ideal for minimal-use systems.
These drives can communicate with the system's motherboard using two different methods. The older and largely obsolete type was Parallel ATA (PATA), which was formerly called IDE (Integrated Drive Electronics). The newer and now standard type is Serial ATA (SATA). This newer type can communicate at a greater rate to make better use of this, the rotational speed of SATA drives was also increased. This means that data can be written and retrieved more quickly, but the drive could potentially fail sooner than it's slower predecessor.
A higher-performance interface is SCSI (Small Computer Systems Interface), configured as SAS (Serial Attached SCSI). Such disks typically have higher performance (higher rotational speeds) than are found on ordinary laptops.