A HDD or Hard Disk Drive is used to store information. It works by using a small disk that has a megnetic field on its surface, the read/write head stores data by using a positive and negative charge with series of ones and zeroes (binary data bits). Normally HDD’s work at around 7200 revolutions per second.
The read write head actually floats on a small cushion of air that is made as the disk rotates. The read/write head sits as little as 3 Nm above the disk as it spins.
SSD’s have no moving parts and use NAND chips to store data in a random order. The seek time is almost eliminated.
Adam H computer blog 