A tab-delimited or comma-separated value (CSV) file are text format files. These files are "just" text and do not include any features, formulas, or formatting from a spreadsheet or database program. If you have a large amount of data to export for use outside your database application, these are excellent formats to use as they work with many other programs.

In order to tell the program where each field begins and ends in an import or export file, the fields must be separated (delimited)
with a character such as a comma (,). This character is called the field separator or delimiter. When the field separator (delimiter) is a comma, the file is in comma-separated (CSV) or comma-delimited format. Another popular delimiter is the tab.

If a field contains the delimiter character within its text, the program interprets this as the end of the field rather than as part of the text. In order to prevent this, it must have a qualifier. The qualifier is placed around each field to signify that it is the same field. The most common qualifier is double quotes ("). Most programs include the qualifier when saving in CSV format. 

For example, the following file is in Excel. You create a CSV file in Excel by clicking File, Save as and selecting CSV (Comma Delimited) in the "Save as type" drop-down.  

Excel file

A comma-separated value file looks like this:
"Mr.","Joe","Brown","1745 E. New St., Apt. 22","Chicago","Illinois","46702"
Double-quotes are wrapped around each field, while a comma separates each field. By including the quotes, the program interprets "1745 E. New St., Apt. 22" as one field rather than two.  

A tab-delimited file looks like this:
Mr. Joe Brown 1745 E. New St., Apt. 22 Chicago Illinois 46702
There are tabs between each field.