CSV, which stands for Comma Separated Values or Character Separated Values, is a text based format that can be used to store information from a spreadsheet or database. Each record is stored on a separate line, and each value is separated by a character like a comma (although other characters can be used). Because of the simplicity of the format it is widely supported, and provides a convenient way to transfer information between different applications. CSV files don't typically support any kind of formatting though.
This guide shows you how to work with CSV files in applications like Excel, and how CSV files can be used to transfer data such as email contacts.