The way data is stored in a file. For example, every document created in Word for Windows is stored as a Word file with special codes to tell Word how the margins are set up, the fonts that are used, and whether any images are included. Each program stores information in its own format. This means it's difficult to read a file that's been created by a different program from the one you are using. To get around this you can either use the Import function, or one of the standard file formats that let you exchange data between programs.