Before we get into the particulars of Apache, let's talk about what a web server is.

What is a Web Server?

A web server is a program that runs on a host computer (also, confusingly enough, called a web server) that serves up web sites. In other words, the web server program sits around awaiting requests from visitors' web browsers for objects it has in its possession, and then sends these objects back for the visitor's viewing pleasure.

Objects that web servers can serve include HTML documents, plain text, images, sounds, video, and other forms of data. These objects may not necessarily exist in static form, but instead are generated on-the-fly by programs run by the server; CGI scripts are the most common of these programs.

Web servers and browsers communicate using HTTP, Hypertext Transfer Protocol, a simple but effective language for requesting and transmitting data over a network. Thus, you'll sometimes hear web servers referred to as HTTP servers.

Web servers come in various shapes and sizes. They run under a variety of operating systems, have varying levels of power and complexity, and range in price from rather expensive to free.

What's so special about Apache?

Apache is:

• Powerful
  Apache's performance and reliability is legendary.
• Feature-Rich
  The Apache server sports a host of features, including: XML support, server-side includes, powerful URL-rewriting, and virtual hosting, to name but a few. We'll be talking about some of these features in future articles.
• Modular
  Looking for a feature not implemented in the core Apache server? Chances are you'll find a module that can add the functionality you need.
• Extensible
  Can't find a module that suits your purposes? Well, as Apache is open source, you can write one yourself. In fact, you can even make changes to the inner workings of Apache. All the information you need is right there in the source code and numerous online resources. Share your patches or modules with the community by making them open source as well!
• Popular
  At the time of this writing, Apache holds a smidge under 60 percent of the web server market. And, yes, popularity does count; help abounds and is only a mailing list or newsgroup posting away.
• Free
  This is one instance where you don't necessarily get what you pay for.

References

Apache is maintained by the Apache Software Foundation's Apache Server Project at http://www.apache.org.