Guides are NPCs located in each town to help the players. They mark important locations in the player's automap and escort low levels to the Depot. The first were implemented in the 2008 Summer Update.
They also gives information about the progress of any World Change by using any of the keywords: Horestis, Mage Tower, Master's Voice, Swamp Fever, Thornfire, Twisted Waters, Awash, Steamship, Horses, Overhunting, Demon War, Hive or Deeplings.
Note: There is no guide in Kazordoon, Svargrond, Farmine and Gray Beach, however they have other NPCs to give you general ideas of where something in located.