Memory hierarchy (fast to slow): registers, cache, RAM (primary, volatile), then secondary storage such as SSDs and hard disks (non-volatile).
1Units
4 bits = 1 nibble, 8 bits = 1 byte
1 KB = 1024 bytes, 1 MB = 1024 KB, 1 GB = 1024 MB, 1 TB = 1024 GB