There will be times when you have to store large amount of log data and other information for your Arduino project, for example a GPS logger. The EEPROM memory of all available microcontrollers are limited in size. The solution is to use flash memories. Carriable version flash memory is available as SD cards, in many storage sizes and form-factors. SD cards and USB flash drives have become an indispensable things in our everyday life.
Even though you could insert an SD card in your phone or computer easily and browse all your files, hardware interfacing an SD card of any type with Arduino or similar development boards isn’t that easy. You could understand it just by reading the SD card spec sheet. The Secure Digital (SD) standard is maintained by the SD Card Association. They standardize and publish all the necessary specifications to manufacture and implement SD cards. Most of the documents are confidential and are only available to manufacturers and software companies. A simplified version (it isn’t that simple) of the SD card specification is available for the public in the SD Card Association’s website.