Unix and Linux term that describes a program that runs automatically without requiring any user interaction. Daemons are designed to help the user by performing tasks commonly done, such as checking for e-mail or in the case of a web server running the HTTP daemon.
For example, httpd (Hypertext Transfer Protocol Daemon) runs in the background of a web server under a number of platforms, including Apache, CERN, and others. It waits for and responds to requests made over HTTP, providing web pages and other files as needed. If you're running a web server and run the top command you'll see httpd requests as they are being made on the server.