In order to play the game you must first choose a server to play on. The servers are used as a way to play the same game multiple times over (with different profiles) with the only difference being the players and the guilds. Some servers have been running for much longer, leading to a lot of players within them being a higher level and more difficult to challenge due to the extended time they have put into their server. Other servers have been set up more recently, so those players haven't reached as high of a level.
There are currently 16 servers available and once you log in, you can choose which server you would like to play on. If you have only been playing in one server and decide you want to try leveling again with new cards, you may choose to play the game from the beginning on another server.
As long as you are logged in, your data across each server is saved separately and treated as a separate entity. This means any money spent on the game in one server is independent and will NOT transfer to any other.You may have reached VIP16 in one server, but you would be VIP0 if you started a new server. Upon re-logging into your original server, you would once again be able to benefit from your VIP rewards.
If you do change server, the game lists "Server recently logged" so that you can easily switch between servers and not worry about forgetting which server you played on previously.