An SSD does functionally everything a hard drive does, but data is instead stored on interconnected flash-memory chips that retain the data even when there’s no power present. These flash chips are of a different type than the kind used in USB thumb drives, and are typically faster and more reliable.
The PC hard drive form factor standardized at 5.25 inches in the early 1980s, with the now-familiar 3.5-inch desktop-class and 2.5-inch notebook-class drives coming soon thereafter. The internal cable interface has changed from serial to IDE (now frequently called Parallel ATA, or PATA) to SCSI to Serial ATA (SATA)
An SSD-equipped PC will boot in less than a minute, and often in just seconds. A hard drive requires time to speed up to operating specs, and it will continue to be slower than an SSD during normal use.
Because of their rotary recording surfaces, hard drives work best with larger files that are laid down in contiguous blocks. That way, the drive head can start and end its read in one continuous motion. When hard drives start to fill up, bits of large files end up scattered around the disk platter, causing the drive to suffer from what’s called fragmentation. While read/write algorithms have improved to the point that the effect is minimized, hard drives can still become fragmented to the point of affecting performance. SSDs can’t, however, because the lack of a physical read head means data can be stored anywhere without penalty. Thus, SSDs are inherently faster.
An SSD has no moving parts, so it is more likely to keep your data safe in the event you drop your laptop bag or your system gets shaken while it’s operating.
SSDs make no noise at all; they’re non-mechanical.
An SSD doesn’t have to expend electricity spinning up a platter from a standstill. Consequently, none of the energy consumed by the SSD is wasted as friction or noise, rendering them more efficient.
While it is true that SSDs wear out over time (each cell in a flash-memory bank can be written to and erased a limited number of times), thanks to TRIM command technology that dynamically optimizes these read/write cycles, you’re more likely to discard the system for obsolescence (after six years or so) before you start running into read/write errors with an SSD.