Can anyone explain to me how a website can change between different web servers? Is this load balancing? or DNS management? Can anyone please explain this technology. To experience this please try istockphoto.co and you will see that their server changes to different server locations with different IP often. You will need to refresh and refresh to see if the server changes.
You can use a physical load balancer to distribute visitors among multiple webservers. Remember you must have a common db server and everything should be in same DC. If you are going to distribute across multiple location, you must setup database replication.
there is no as such majic, just depends on new and old server specs, and the tools they are using, nothing else
Load balancing is only useful for super-high traffic sites. A single dedicated server is more than enough in most common cases.