Servers or Realms are the different worlds you can play on in the game. Some of these realms are exclusive to the acclaim client, while others are exclusive to the european client. There are two types of servers in this game: PvE and PvP. The more hardcore PvP players are generally found on the PvP servers, though you will still find PvP activity on PvE servers.
Note that the server structure of the game can change at any given moment, new servers may rise or old servers that are nearly empty might get merged with other servers. An example of the latter is the merging of the european french, german and international servers into just two international servers in order to get a larger amount of players on each. This action was wished for a longer amount of time by the players.