When you want to print a page of text, your computer has to present the page to the printer in a way it can understand. It uses the print drivers to do this - to translate the information into your printer's language. But once the print driver translates all that information, your computer has to feed it to the printer at a certain rate. So it uses a 'spooler'.
The printer spooler collects the pages delivered by the printer drivers, and stores them on your hard drive in a 'printer queue'. Once the spooler is ready to deliver the job, it starts feeding it to the printer itself.
When you run a print job on Windows XP or Windows Vista, you should see a small picture of a printer pop up on your notification bar (the bottom, right hand corner of your screen next to the clock). If you double click on this, the print spooler service gives you a screen to see the different print jobs, how large they are, and what order they're set to print. You can right-click on any one of these jobs and cancel, pause, or restart the job.
The printer spooler service runs in the background, and when working right, doesn't use many resources. If you look at the running processes in your task manager, the spooler service is named "spoolsv.exe". Generally, it will only use up to 8 megabytes of memory, unless your computer is in the middle of a print job. It shouldn't be using ANY cpu while you're not printing. It shouldn't go above 2% when you are printing.