One way to explain print spooling is this:
Lets imagine that you have created a document on your computer that you want to print.
If it was the case that when you selected the print command, your word processing program sent that data directly to your printer, you would have to wait quite a long time for it to finish sending the data stream.
This is because before you could get carry on using the word processor, it would be sending a character to be printed, and then waiting for the printer to reply back saying "Ok, I have printed that character, send me another".
Another problem would be that you could only ever have one program trying to print at the same time. Think about printing a massively long document from your word processor and while waiting for that to complete, you decided to print a spreadsheet at the same time.
So, the job of a print spooler is to a) pretend to be a real printer to programs that want to print, and b) send the data to be printed to the actual printer itself.
As far as your word processor is concerned, the print spooler IS your printer. As the print spooler is simply another program rather than a physical device, it can accept your characters and graphics almost as fast as your word processor can throw them out. This means that you perceive that the printing has completed a great deal sooner than it really has.
The other thing the print spooler can do is accept a stream of print from more than one program at once - for instance your document and your spreadsheet.
The spooler will keep everything in order and look after sending the data to the very slow printer without holding up your work.
Phew - I hope all the above gobbledegook helps :)