The document discusses physical storage media used in database systems, including magnetic disks, flash memory, optical storage, and tape storage. It describes the storage hierarchy from fastest but most expensive primary storage (e.g. main memory) to slower but larger secondary storage (e.g. magnetic disks) to slowest but largest tertiary storage (e.g. tape). It also discusses techniques for improving disk performance and reliability, including disk striping, mirroring, and various RAID levels that use parity bits.