.csv files

From EU3 Wiki

.csv files are text files consisting of tabular data in columns separated by semicolons.

They can be edited in a text editor, or by using a spreadsheet software like Microsoft Office's Excel or Open Office's Calc. Using spreadshetts may cause corruption if you do something wrong during import/export, so it is adviseable to at least keep a backup fi you are not 100% sure on what you are doing.