You have seen www , http, https — now why do you see www2 or www3 in many URLs ( web addresses ), what exactly is this www2 or www3 ? For ex www3.google.com What is the difference between www and www2 or www3 ?
In the DNS hierarchy www3 or www2 is simply a subdomain.
Many businesses and complex websites uses a sub-domain of www2 or www3 to separate data or certain services from the their main website ,yet utilizing the advantage of using the original site domain name.
According to wiki : Some websites use different subdomains to point to different server clusters. For example, www.example.com points to Server Cluster 1 or Datacentre 1, and www2.example.com points to Server Cluster 2 or Datacentre 2, etc.