Realm

A realm is an instance of the World of Warcraft (WoW) game world. Realms are hosted on physical devices called servers (for this reason, the terms "realm" and "server" are often used interchangeably). This prevents any one realm from becoming too crowded and overwhelming its associated server, provides for better connection times by allowing players to connect to local servers, allows for localized communities based on language, and to appeal to different play-styles for different users.

To play WoW, you must first choose a realm to play on. You then connect to this realm over the Internet. Blizzard has servers in the US, Europe, China, and Korea.

Types
According to the North American realm status page, there are 241 total NA realms. Of these 113 are Normal (PvE) servers, 105 are PvP, 17 are RP, and 6 are RPPvP. For Europe, there are 109 English speaking realms, 87 German, 37 French, 11 Spanish, and 19 Russian, for a total of 263 realms. This breaks down into 99 Normal (PvE), 129 PvP, 20 RP, and 15 RPPvP servers.

Player versus environment
Abbreviated PvE, the player versus environment realm allows for a more relaxed style while in the world. Depending on the location within the world, it is more difficult to initiate player versus player combat. This realm is also referred to as a "normal" realm.

Player versus player
Abbreviated PvP, the player versus player realm differs from the normal realm. Entering any contested or enemy territory automatically flags the character being used for PvP combat, and the character cannot be unflagged while in these zones. The player will remain flagged for five minutes after having left a contested or enemy territory,

Roleplaying
Abbreviated RP, the roleplaying realm features the PvE playstyle with roleplying rules, which apply to behavior and naming.

Roleplaying player versus player
Abbreviated RP-PvP, the roleplaying-player versus player realm features the PvP playstyle with roleplaying rules, which apply to behavior and naming.

Test
For the adventurous, there is also a Test Realm that is periodically available in which players can try experimental features in development before they get released as general patches. Test realms often exist in both PvE and PvP varieties.