Often called storage or memory, is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
There are three main categories of storage devices: optical, magnetic and semiconductor. The earliest of these was the magnetic device. Computer systems began with magnetic storage in the form of tapes (yes, just like a cassette or video tape). These graduated to the hard disk drive and then to a floppy disk.
Internal: Any sort of permanent storage which is internal to a computing device. Stuff like hard drives, SSDs, soldered on flash disks, etc.
External: Similar (or even exactly the same thing) as above. Just that it’s removable without needing to open the device.
Direct-Attached: deals with local disks in servers. The networking and high-performance interfaces still do not allow building diskless servers for all solutions. Built using a standardized protocol, Fiber Channel, SANs are designed for high-performance block-mode access to raw devices. SAN-attached devices have no concept of file systems, a notion that remains at the operating system. After being placed in complex and high-end infrastructures, SAN are now making their way into the smaller small and medium business (SMB) market.
Much of the upcoming challenge of the this industry is to propose simple ways of managing relatively complex storage components. The NAS fundamental is the utilization of a data network protocol, IP (and more precisely, TCP/IP) to a file-share protocol. NAS made its way from file-sharing appliances, which rapidly evolved into mature storage components. Each of these storage approaches has pros and cons, all of which depend on the objectives one is pursuing.