A few selection criteria

Language
The language of the server corresponds to the official language used to talk in game with the other players (chat). However, when connecting you can select the “game interface language" but this has nothing to do with the “server language”. Indeed, the language of the interface corresponds to the language in which the name of the monsters, weapons etc. and the NPC dialogs will be displayed.
Accounts
On one single account, you can create up to 5 characters. You can share your Friend List (up to 30 friends) with all the characters of your account no matter the server they're on. In fact, they are accounts you listed as friends and not as characters. The Friend List is shared with all the servers but you’ll only see and be able to communicate (chat) with people on the same server you're on.
Servers
When you’re on one specific server, the content of your safe is divided between all your characters, no matter which bank you’re at. This way, you can transfer your items from a character in the bank of Astrub to another character in the bank of Bonta (or on the same bank) without any problem. However, you can not share anything between 2 characters on 2 different servers (neither characters, nor items or Kamas).
However, you can name your characters with the same name on different servers (provided no one else already chose it on the said server).
Server evolution level
The older a server is, the longer its life's been. Therefore its population is experimented both in fights and professions and crafted or dropped items are of a higher level and more widely spread.
However, the currency loses its value as time passes and you need more and more Kamas to buy items that you'll find at a cheaper price on a more recent server.