If the website has a lot of traffic, they usually setup some sort of system where it spreads the traffic across multiple servers. So sometimes you will get www2 www3 www15 etc... it just means you are hitting a different webserver...
you can use nginX for content distribution. I usually do it using different DNS-A-Resource records for the same host, so machines will be redirected randomly between the servers and all of them still have the same hostname to a user. causes less confusion and is way easier to setup EDIT: Using different A-Records also makes it easier to redirect on geolocation(ie users from europe get redirected to a european server, us visitors get redirected to a us-based server and so on)
LOL @ you. Anyway, back to the question: BIG sites which has tons of traffic, even best dedicated servers find it hard to be handled. Because the resources used are just too much. So what they do is, have multiple servers run the site. So depending on the server you are connected you get to see www, www2 etc.