Different URL

So, i have seen several sites where the URL starts with www1. or www2. 
And im just curious about it. 

They are subdomains: http://en.wikipedia.org/wiki/WWW2

Also I don't know if you can delete your own posts but it appears you've posted the same question twice.

Thanks m8 <3.

Sorry about the dubble post, i will see if i can delete the other one in some way :D