For the Alliance EU players they are:  Silvermoon, Ravencrest, Antonidas.
Realm is the World or server where you will level up your character. Even though interaction between characters from different Worlds is possible, for a greater immersion in the atmosphere of the game it is better to choose the most densely populated ones. For the Horde EU players they are: Draenor, Kazzak, Twisting Nether. For the Horde US players they are: Area 52, Illidan, Thrall. For the Alliance EU players they are:  Silvermoon, Ravencrest, Antonidas. For the Alliance US players they are:  Stormrage, Sargeras, Proudmoore. If someone you know is already playing the WoW or you are just going to start a journey with someone you know, then choose the same server.