CSV, or comma-separated values, is a common way to store tabular data. You can often import and export CSV files with spreadsheet and database software. Not all CSV files have geospatial data: they are usually just tables. Placemark supports different kinds of CSV files:
TSV files, which use tabs to separate columns, and DSV files - delimiter-separated files, which can use other separators like ; - are functionally equivalent to CSV files. Importing those files is supported in the same way.
CSV has no standardized way to represent data types other than points, so CSV files are not traditionally used for data like lines or polygons.