.csv files are '"Comma-Separated Value" files.
Most spreadsheet applications/programs (such as MicroSoft Excel) can import .csv files. Once imported, they can then be graphed.
.csv files are ASCII text files, with the data in columns separated by commas. And, line is a row--just like in spreadsheets (columns and rows).
You can also look up many filename extensions using your preferred Internet search engine and get pretty good descriptions, and sometimes even the names of the applications which created them or can read/open them.
Hope this helps!