The Common UNIX Printing SystemTM (or CUPS) is the software which allows you to print from within applications, e.g. from this web browser.
CUPS converts the page descriptions produced by the application, e.g. "Insert Paragraph", "Draw Line" etc., into data which can be read by the printer, and then sends this information to the printer.
With the appropriate configuration, CUPS can use printing devices via the following connections:
Default Paper Size: Specify a default paper size for print jobs.