Apache is the leading HTTP server software on the web. It was originally developed and released as an open source set of installations and configuration patches for the NCSA httpd process in 1995, and has been rewritten from the ground up at least twice since then. It is the industry standard, and more web servers run on Apache than all other web services combined; it is the back-end software that runs over 60 per cent of all the web sites out there.
Advantage: It's Open Source
Open source software is software written by amateurs for the joy of solving problems. The real benefit of open source as a development model, particularly as it applies to Apache, is that many eyes looking over the source code and many people contributing to the source code means that bugs get fixed fast, and they get fixed constantly, as they're found. New services in open source software tend to be slower in coming than for Internet Information Server, in large part because there isn't a marketing department trying to find new things with which to drive sales. The end result is that Apache, running on a Linux or Unix server, is robust, stable and can be very secure.
Advantage: It's Free
All open source software is distributed under license conditions that make the source code freely available. This means that you can download the Apache server modules (and even run it under Windows) without having to pay a licensing fee. This is particularly important for web-hosting companies and Internet service providers that have to constantly increase server capacity. As a case in point, Google's server farms use tens of thousands of Apache installations, saving the company a lot of money in licensing fees.
Disadvantage: It's Written By Geeks, For Geeks
Most Apache installations go off without a hitch or hiccup, and most Linux distributions have Apache bundled in as part of their installation image, already pre-configured and ready to run. However, if you go off the beaten path, or run into a configuration problem, there's no central office to turn to for technical support, only web forums full of people who had the same problem, and hopefully documented their solutions to them. The corollary of this is that if you have to fix something, you'd better be comfortable with command line prompts and the somewhat cryptic Unix command sets, because there is absolutely no handholding or set-up wizards for beginners.