Hello,
All the information above is excellent.
A few extra things to consider:
Regular Hard Disk Drives use what's called a "platter" inside of the HDD. The platter is a circular magenetic disk that contains information on "tracks" which are circular (and go around the platter). By defragging a HDD, you put the information in a relatively "straight" line/same area on the platter. If a HDD is heavily "fragged" all that information (OS, program files, etc) would be in random places on the tracks and on the platter, slowing the loading process as the head reader in the HDD has to "travel" further than if the system was de-fragged.
By design, SSD's install information in a "fragmented" way. However, each nand cell can be read/written to, so even though the information is "fragmented", it will still read much faster than a HDD. For this reason you should not "defrag" a SSD. It is meant to be fragmented. Forcing an SSD to defrag would make it continuously read/write and move the data pointlessly, basically lowering the lifespan of the SSD.
SSD's usually contain anywhere from 4-16 flash chips on them - and each chip can be read/written to, versues a hard drives single head that can only read information in a "straight line" on the platter.
Hope this helps.