What is a CSV File
A CSV is the shortened name of a comma separated values file. This type of file allows data to be saved in a table structured format.
CSVs can be used with any spreadsheet program and actually look like ordinary spreadsheets so you can use them in programs like Microsoft Excel or Google Spreadsheets. They differ from other spreadsheet file types in that you can only have a single sheet in a file (rather than many sheets) and they don't save cell, column, or row styling, or formulas. If you want to do any of these advanced things with your CSV file then you can easily turn it into an Excel workbook by opening it in Excel, going to File -> Save As, and then selecting 'Excel Workbook'.