Horde PvE Horde PvP Alliance PvE Alliance PvP

Color Coding
To determine the status of other players, check the color of their character name (the one that appears over the character's head).
Green - Friendly character, PvP mode enabled.
Blue - PvP mode disabled; you cannot attack this character.
Yellow - Enemy character, PvP mode enabled; this character cannot attack you unless you attack him first or activate PvP mode in another way (see below).
Red - Enemy character, PvP mode enabled; this character can attack you, and you can attack this character.
PvP Mode Activation: Any Realm
On all realms, actions that will trigger PvP mode are:
Attacking a player character (PC) of the opposite faction who is already in PvP mode
Helping a player character (PC) of your own faction who is already in PvP mode, by using a beneficial spell or ability on them
Attacking a non-player character (NPC) guard or civilian of the opposite faction, such as a town guard
Entering a battleground
Entering an enemy capital city
Entering a special PvP area, such as Gurubashi Arena in Stranglethorn Vale - this will activate a free-for-all PvP mode where you can even attack members of your own faction!
The PvP mode will stay enabled until five minutes after the last action that triggers the PvP mode activation.

PvP Mode Activation: Player vs. Player Realms
The following additional actions will also trigger PvP mode on PvP realms:
Entering a contested zone
Entering an enemy zone
The PvP mode will stay enabled until five minutes after the character has left the zone or battleground.

PvP Mode Activation: Manual Activation
By typing /pvp, you can activate PvP mode for your character manually. Note that PvP mode will stay enabled until you manually turn it off by typing /pvp again. After you manually deactivate PvP mode, you still need to wait five minutes until your character reverts back to PvE mode.

Choosing Your Realm
After you've decided what realm type is best suited for you, the next step is choosing which particular realm of that type to start your adventuring career on. The Realm Status page has a list of all realms, with the type and population of each realm displayed next to their names.
Another consideration when selecting your realm is which Battlegroup your realm is part of. This is a concern if you intend on participating in PvP with the honor system or arena system. It's advisable to do some research to make sure you're happy with the mix of servers in your destination Battlegroup. Some Battlegroups have more Normal servers while others have more PvP servers. A few of the Battlegroups are very challenging and are known to have intense PvP with top players.

Remember that your character will most likely spend his or her entire career on the same realm, so choose wisely! (You can later transfer off the realm using Paid Character Transfer if you really need to.)