Web Server:
Also called HTTP Server. A computer where Web sites and Internet sites are stored. The server software uses HTTP to serve HTML documents, associated files, and scripts when requested by a Web browser.
From: www.dsl-experts.com/broadband_glossary_w.htm

Although it might seem that a web server is a machine, it is really a process running on a machine that serves HTTP content to web browsers on client machines. This is important because you can actually put several web servers on a single machine as long as you designate specific ports through which traffic will travel. Along the same lines, there are also FTP servers, mail servers, and so on, each of which handles a specific type of traffic.
From :www.nas-san.com/gloss.html