A Web server is a program, which allows Web browsers to retrieve files from computers connected to the Internet. The Web server listens for requests from Web browsers and upon receiving a request for a file sends it back to the browser. It is a computer on the Internet that contains web pages, and that delivers those web pages to computers that contact it.

Web servers can be any kind of computers. They can be Windows-based PCs, Macintoshes, or computers that run the Unix operating system, or a popular variant of Unix called Linux. These servers run software called web server software the software that speaks HTTP.

There are many Web server software applications, including public domain software from NCSA and Apache, and commercial packages from Microsoft, Netscape and others. Every server on the Web has its own unique number, called an IP address.

There's no way to remember every single number of every single website, so the server also has a domain associated with it, such as google.com. When you type in the address of the website, special servers on the Internet called DNS servers translate that address into an IP address, and your web browser goes to the right location.

Web servers often come as part of a larger package of Internet- and intranet-related programs for serving e-mail, downloading requests for File Transfer Protocol files, and building and publishing Web pages. Considerations in choosing a Web server include how well it works with the operating system and other servers, its ability to handle server-side programming.

Most Web servers are run on one of three Operating Systems:

- Linux

- Windows

Following are the popular Web Server Software in the market.

This is most often the server of choice for corporations that are using Unix Web server machines. The Sun Java Web Server offers some of the best of both Apache and IIS in that it is a supported Web server with strong backing by a well known company. It also has a lot of support with add-in components and APIs to give it more options.

This is a good server if you are looking for good support and flexibility on a Unix platform. Sun Java System Web Server is a web server designed for medium and large business applications. Sun Java System Web Server builds on the earlier Sun ONE Web Server, iPlanet Web Server, and Netscape Enterprise Server products.

Sun Java System Web Server is available on all major operating systems, supports JSP and Java Servlet technologies, PHP, NSAPI, CGI, and ColdFusion.

- Apache

Apache is free software distributed by the Apache Software Foundation.

Apache is generally recognized as the world's most popular Web server. Originally designed for Unix servers, the Apache Web server has been ported to Windows and other network operating systems The Apache Web server provides a full range of Web server features, including CGI, SSL, and virtual domains. Apache also supports plug-in modules for extensibility. Apache is reliable, free, and relatively easy to configure.

Apache supports a variety of features, many implemented as compiled modules which extend the core functionality. These can range from server-side programming language support to authentication schemes. Some common language interfaces support mod_perl, mod_python, Tcl, and PHP. Popular authentication modules include mod_access, mod_auth, and mod_digest. A sample of other features include SSL and TLS support a proxy module, a useful URL rewriter , custom log files and filtering support.

- Microsoft IIS

Microsoft Internet Information Services or IIS is one of the most popular commercial Web servers available for the Windows Server OS. IIS offers functionality and scalability on the familiar Windows operating system. IIS is the server that integrates directly into the Windows server edition OS. It allows you to run ASP.Net and other applications.

Some of the feature IIS is providing are Common HTTP Features, Application Development Features, Health and Diagnostics Features, Security Features, Performance Features, Management Tools, Windows Process Activation Service, File Transfer Protocol Publishing Service Features, Simultaneous Connection Limits.