They are a computer component used to store your data permanently. This means that the data is not erased when the unit is turned off as it happens in the ones stored by the RAM memory. SSD: they are an alternative to hard drives. The big difference is that while hard drives use moving mechanical components, SSDs store files in microchips with flash memories interconnected with each other.
Source conceptshardware-ruth.blogspot, xataka
HARDWARE GLOSSARY 