What is OS spool in printing?

As we know, spool request was output to on a particular device, that is, output request created. The data from spool request was converted to printer language. Between SAP system and printer, there are OS Spool and Print Server. (Refer to TADM12_2, 2005Q4 version, Figure 185). I am confused the concept of OS Spool and Print Server. Where is OS Spool, and where is Print Server? Please advise. Thanks

James