Memory and storage trail
Primary storage is the fastest type of storage in the computer. It includes the system RAM (Random Access Memory), and also the ROM (Read Only Memory) which contains the code needed to boot the system. Primary storage is accessed directly by the CPU bus, which makes it very fast - it only takes a few clock cycles to access data. However it is relatively expensive compared to hard drive storage, so most computers use a combination of both.
Computer memory is fast, but quite expensive. It is also volatile (the data is lost when the computer is switched off). Computers use secondary storage to store larger amounts of data, and also to keep data permanently, so it isn't lost when the computer is switched off. The most common form of secondary storage for PCs is the hard disk drive (HDD). For mobile devices, Flash memory is more suitable. Typically, all applications and data are kept in secondary storage. The code and data is copied into memory when the application is opened.