A web server is the combination of computer and the program installed on it. Web server interacts with the client through a web browser. It delivers the web pages to the client and to an application
by using the web browser and he HTTP protocols respectively. We can also define the web server as the package of large number of programs installed on a computer connected to Internet or intranet for downloading the requested files using File Transfer Protocol, serving e-mail and building and publishing web pages. A web server works on a client server model. A computer connected to the Internet or intranet must have a server program. While talking about Java language then a web server is a server that is used to support the web component like the Servlet and JSP. The web server supports only component like servlet and jsp.
If you want to know the difference between Web Server and Application Server, please visit the following link:http://www.roseindia.net/web-application-server/tomcat/web-server-and-application-server.shtml