Well, it's basically a completely new printing system based on the Internet Printing Protocol ("IPP") that supports PostScript and non-PostScript printers and a variety of different file formats to make your life easier.
CUPS provides all of the normal printing commands ("lpr", "lp", etc.) - you still use "lpr" to print from Netscape, etc. However, these commands take on a new life with CUPS - instead of bringing up an application everytime you want to print, you can print most images, PDF files, etc. directly. CUPS figures out the type of file and runs any necessary filters to format it for the printer. Have a file that CUPS doesn't handle? No problem, just add a filter and CUPS will handle it, for any printer you have.
Printer drivers are provided for PostScript and HP PCL based printers. We're hoping that the filters provided with CUPS (including a PostScript RIP based on GNU GhostScript) will encourage independent developers and printer manufacturers like EPSON to start developing drivers that use CUPS. Only time will tell.