The form tag is something like this:
<form name="form" action="form.php" method="post">
<input name="field1">
</form>
On the previous example "form.php" is the action file, it is what will read your form. It need to be coded with a server side language and is ran from the server.
<?php
extract($_POST);
echo $field1;
?>
To save the data on your hard disk you will need to save it on the server first, and then you can download the file to your computer.
To create a file with the data retrieved from a form, you will also need a PHP file:
http://www.scriptingok.com/forum/t5_964-Create-a-file-from-a-textareaPost again if you need more help.