The purpose of systen storage is to store all your documents, downloads, programms, etc so you access them everytime you use your computer. There is two types of storages you can have in your computer HDD (Hard Disk Drive) or a SSD (solid State Drive) or you can use both it is up to you. The diiference between the two storages are, SSD is like a memory stick where it has no moving parts, information is stored in microchips this allows you to access information much faster than a HDD. A HDD uses a mechanical arm with a read/write head to move around and read information from the right location on a storage platter. This takes more time to happen and is slower compared to a SSD which does it nearly instantly because all it has to do is access the microchips that have all the date stored and sorted.
