Web Server

Web server is a computer where the web content is stored. Basically web server is used to host the web sites but there exists other web servers also such as gaming, storage, FTP, email etc.
Web site is collection of web pages whileweb server is a software that respond to the request for web resources.

Web Server Working
Web server respond to the client request in either of the following two ways:

Sending the file to the client associated with the requested URL.
Generating response by invoking a script and communicating with database

Key Points
When client sends request for a web page, the web server search for the requested page if requested page is found then it will send it to client with an HTTP response.
If the requested web page is not found, web server will the send an HTTP response:Error 404 Not found.
If client has requested for some other resources then the web server will contact to the application server and data store to construct the HTTP response.

Architecture
Web Server Architecture follows the following two approaches:
Concurrent Approach
Single-Process-Event-Driven Approach.

Concurrent Approach
Concurrent approach allows the web server to handle multiple client requests at the same time. It can be achieved by following methods:
Multi-process
Multi-threaded
Hybrid method.
Multi-processing
In this a single process (parent process) initiates several single-threaded child processes and distribute incoming requests to these child processes. Each of the child processes are responsible for handling single request.
It is the responsibility of parent process to monitor the load and decide if processes should be killed or forked.

Multi-threaded
Unlike Multi-process, it creates multiple single-threaded process.

Hybrid
It is combination of above two approaches. In this approach multiple process are created and each process initiates multiple threads. Each of the threads handles one connection. Using multiple threads in single process results in less load on system resources.

Examples
Following table describes the most leading web servers available today:

S.N.    Web Server Description
1    Apache HTTP Server
This is the most popular web server in the world developed by the Apache Software Foundation. Apache web server is an open source software and can be installed on almost all operating systems including Linux, UNIX, Windows, FreeBSD, Mac OS X and more. About 60% of the web server machines run the Apache Web Server.

2.    Internet Information Services (IIS)
The Internet Information Server (IIS) is a high performance Web Server from Microsoft. This web server runs on Windows NT/2000 and 2003 platforms (and may be on upcoming new Windows version also). IIS comes bundled with Windows NT/2000 and 2003; Because IIS is tightly integrated with the operating system so it is relatively easy to administer it.

3.    Lighttpd
The lighttpd, pronounced lighty is also a free web server that is distributed with the FreeBSD operating system. This open source web server is fast, secure and consumes much less CPU power. Lighttpd can also run on Windows, Mac OS X, Linux and Solaris operating systems.

4.    Sun Java System Web Server
This web server from Sun Microsystems is suited for medium and large web sites. Though the server is free it is not open source. It however, runs on Windows, Linux and UNIX platforms. The Sun Java System web server supports various languages, scripts and technologies required for Web 2.0 such as JSP, Java Servlets, PHP, Perl, Python, and Ruby on Rails, ASP and Coldfusion etc.

5.    Jigsaw Server
Jigsaw (W3C's Server) comes from the World Wide Web Consortium. It is open source and free and can run on various platforms like Linux, UNIX, Windows, and Mac OS X Free BSD etc. Jigsaw has been written in Java and can run CGI scripts and PHP programs.