The main reason for the prevalence of hard disc drives (HDD) is their low cost, large storage capacity, and relatively fast data writing.
Each disk contains a metal or glass plate with a thin layer of magnetic material on top. The plates are rotated, most commonly at a speed of 7200 turns per minute (for server and high-performance models up to 10 or 15,000 rpm, for some low energy and 2.5" laptop discs only 5400 rpm). They are rigid in design - hence the name "hard" drive. Conventional 3.5" drives for desktop PCs feature 4 plates and 8 read heads (one head on each side of the plate).
SSD (Solid State Drive) drives are purely electronic - they store data in flash memory NAND - and do not work with any moving parts. As a result, they have a shorter reaction time but are less prone to mechanical damage. They are used mainly in laptops.