CSV stands for Comma Separated Values.
This is an exchange format for spreadsheet programs including Excel which is part of Microsoft Office files. It is basically just an ordered ASCII file with each value delimited with commas on one line and each line in the file representing a row in the spreadsheet. It is often the format used by outside programs to prepare a file to be imported to a spreadsheet.
In practice the comma is the preferred separator but actually any character not otherwise used in the file can be used. Sometimes a tab (ASCII value 09) is used which is often referred to as Tab delimited data. Using a delimiter other than a coma requires the accepting program to have the ability to be told what the character is. Even a space can be used as a delimiter.