Primary Storage, refers to computer storage which is directly access or manipulated by Central Processing Unit (CPU) or Processor. This type of storage includes Random Access Memory (RAM) and fast Cache storage.
Primary Storage is generally very fast but expensive storage, since modern processors are very fast and required faster primary storage to keep up with faster response time of modern applications.
Primary storage is generally volatile means data in the storage gets lost in your turn of computer or device.
Secondary Storage, refers to storage devices capable of storing long term persisted data. These devices include Hard Disk Drive (HDD), Solid State Drives (SSD) or Flash Drives.
These storage mediums are non-volatile which mean it can keep the data persisted even if power is turned off. Secondary storage is used by programs to store data when you save for example in document processor program when you open document it loads it into primary memory i.e. RAM but after making changes to document and hit save it get persisted into secondary storage i.e. Hard Disk.
Secondary storage is slower than compared to primary storage and hence cheaper than well. Solid-State Drive is significantly faster than traditional Hard Disk Drives and helps boost system performance.