Apple - What is CUPS on port 631?

From the CUPS website:

CUPS is the standards-based, open source printing system developed by Apple Inc. for OS X and other UNIX®-like operating systems.

On OS X it provides local printer services as well as shared printer services for other Macs to access your printer via the service if the printer doesn't have its own sharing capabilities. CUPS has become fairly ubiquitous across the Linux/UNIX contingent of OSes as the print server if choice.

That open port you found is because printer sharing is turned on on your machine. You can close port via System Preferences > Sharing and uncheck Printer Sharing.

Actually CUPS (the Common Unix Printing System) was an open source project that Apple bought. Contrary to what is listed at CUPS.org, CUPS existed long before Apple bought it.

I understand that CUPS remains Open Source and that (some?) code contributed by Apple works it's way back into the Open Source version available to and used by other O/S'

Note that CUPS is an excellent (though somewhat complicated) web interface to the printing subsystem on the Mac. In some rare cases it is possible to add and manage printers that do not offer a printer driver for the current version of Mac O/S but do offer one that works with CUPS.

The open port is for internal (localhost) services only. It should not be visible to a remote host, e.g. via the 'nmap' utility.