Common Unix Printing System™.
CUPS is a replacement for lpd. It's a print spool system; Programs write printable data to CUPS daemon via "lp" program (or via lpd emulation using "lpr" command), and the daemon prints them to printer when it becomes available. Printing over network is supported (using ipp protocol).
CUPS daemon processes the data in two possible ways: Either it prints the data by converting it to either Postscript or PCL using its built-in filter, or passes it to the printer directly in raw format if so desired. The latter is handy in case you have an inkjet that has no clue of either PCL or PS - you can usually hope GhostScript has output backend for your printer, then, and pipe its output to lp.
What makes CUPS so cool compared to lpr is that administration can be done with any web browser; Simply point web browser at http://print.server.address:631/ and log in, and follow instructions. No more messing around with printcap! (Tip: that port really needs to be firewalled at some points, and it's a non-SSL connection so it's best done on the printer server, lest someone sniffs your root password. =)
CUPS home page: http://www.cups.org/