HHDs or Hard Disk Drives are storage devices that function via magnetic storage. The data is stored on rotating disks coated with magnetic material, and this is how they can be easily identified. The data is read and stored via actuator arms. HDDs are cheaper than flash media, however they are slower and less stable and are therefore phasing out in popularity.
SSD, Solid State Drive, or Flash Memory, is a newer technology. The data is stored in integrated circuits or cells that can be accessed instantly rather than needing to spin like HDDs. SSDs can also be made into a much wider variety of sizes and applications. They are not susceptible to magnetic damage and, due to fewer moving parts, SSDs last longer. Because of this durability, SSDs have finer requirements for destruction than HDDs