One thing that people don’t often think about is that there’s more than one web server program out there. It’s not really visible to visitors, since they all do basically the same thing, but there are lots of servers available, and they’re all quite different in the way they work. There are three main groups:

Apache. The open source Apache software is the most popular server software out there, with around 70% of the market share.

Microsoft servers. Microsoft are responsible for the various versions of IIS (Internet Information Server) and PWS (Personal Web Server), which altogether have around 20% of the market.

Sun servers. Sun produce lots of servers, most notably the Netscape-branded ones. The market share of these servers depends on whether you count all sites (making it 3%) or just the actively maintained ones (in which case it drops to less than 1%).

Source for statistics: netcraft.com.

Other servers available are mostly ’simple’ servers that don’t have all the somewhat unnecessary features of these servers, such as thttpd (the ‘t’ is for tiny or turbo). There are literally hundreds of them, but they have mostly negligible market share.