A Unix term used to define programs that run in the background. Other operating systems use different names, "service" and "server" being the most popular. A web server software such as Apache is a daemon that transparently runs in the background and serves document requests. Another example is MySQL, a relational-database server that serves database queries coming from other programs such as PHP scripts. As you see, MySQL could not serve other programs if it was not running in the background. Another example is a print spooler that queues documents and send them to the printer one by one so you do not have to wait the printer finish the previous page before sending the next page.