the parser and the callback function are two steps in the import process.
The parser reads the feed line by line, thus item by item.
Then the item should go thru the callback function generic_cb, this funcion maps the fields in the item from the feed to the fields in the database based on the mapping on the right of the feed configuration. You use a different function to manipulate the data.
Back to the parser.
the component can handle almost any CSV formatted feed with a a header row, these is where you use the CSV parsers.
Some network have XML formatted feeds as well, since all XML files are different each format needs a different parser, the component includes parsers for some (dutch/european) networks.