This format can contain spreadsheets data separated by commas. It is often used for the import and export of contacts between services and messaging softwares (Thunderbird, Gmail, etc.) And sometimes old documents are in this format also.
Name : Comma-Separated Values
Mime : text/csv
Licence : Open
Type : Tableur/texte
Free softawres to open csv : LibreOffice Calc
Non free softwares to open csv : Microsoft Excel