In the game manual
And I quote from it directly
"Choosing a Realm If you want the thrill of fighting other players, and prefer the challenge of playercontrolled opponents over computer-controlled ones, then the PvP realm is for you. However, if you choose this realm, be aware that in most areas of the world you risk getting attacked by higher-level enemy players when you least expect it. Not only might they be too powerful for you, but they’ll also strike when you are at your most vulnerable. For example, they might attack you while you are in the middle of a quest or a monster fight. Interference by opposing players is a constant reality you must accept on a PvP realm. If you want to restrict your gameplay to computer-controlled challenges, and want the freedom to enter PvP mode only at your own choosing, then the PvE realm is for you. For more detailed instructions on the realms, read the World of Warcraft website "