The description of how a file is represented on a computer disk. The format usually corresponds to the last part of the file name (‘extension’), e.g. a file in CSV format might be called schools-list.csv. The file format refers to the internal format of the file, not how it is displayed to users. E.g. CSV and XLS files are structured very differently on disk, but may look similar or identical when opened in a spreadsheet program such as Excel.

Source: ODH