In the vast majority of cases, computers load their programs and data from an internal HDD (Hard Disk Drive).

Hard Disk Drives are basically boxes of rotating disks which use a stylus to read data from the disks. They are fitted internally to PCs and laptops and need to accelerate to operating speed for data to be read.

A 5400rpm hard disk will need to accelerate to 5400rpm, the stylus will need to move to the disks, find the required files and retrieve them.

The above process obviously causes wear, generates heat, generates noise and uses power. Because SSD is a Solid State Drive, it uses less power, it is virtually silent, generates virtually no heat and there is no wear (there are no moving parts).

SSD units (Solid State Drives) are basically boxes of memory. This is why a quality SSD is far more reliable, silent, generates minimal heat, uses minimal power and is always  significantly faster than a hard drive (the drive does not need to accelerate to operating speed and the stylus does not need to search the drive for data).

Overall, an SSD drive is usually the best single upgrade that can currently be made to most computers (tower PCs, desktop PCs and laptops), followed closely by memory upgrades.

Although the capacity of SSDs tends to be lower than Hard Disk Drives, in most cases this is not an issue.