Well it depends on what you mean. For the most part the internet is a very diverse enviorment, a loses contected series of machines connected from all over the world. Now there are some systmes that are key, such as DNS serveres that allow people to get to access systems by common names instead of IP address. DNS servers can be run from many types of OSes, but many of the largest are indeed UNIX systems. If you are interested in what type of systems are running web servers, you may want to take a look at...
http://news.netcraft.com/
Here you can look up the server type that is behind a given site. While it can not be taken as absolute truth, it can be handy information for the curious.
Enjoy,
