Manual

If you click on an NPC or monster, three objects appear on your screen informing
you about the targeted creature. There is an NPC portrait at the top of the screen,
a tool tip in the bottom right of your screen, and a selection circle around the
creature. If the creature is aggressive, than that means it will attack you as soon as
you approach close by. You can tell if a creature is aggressive by the color of its
selection circle, tool tip, and name bar in its portrait. If the color is red, then the
creature is aggressive. If the color is yellow, then the creature is passive. That
means that it will only attack you if you attack it first. If the color is green, then the
creature or NPC is friendly and cannot be attacked. Finally, if the color is blue, then
the selected target is another player character.
The tool tip will also appear whenever you mouse over a creature or NPC in the
game world. Use it to your advantage to see if it is safe to approach nearby
monsters and non-player characters.
Hostile NPCs
Non-player characters that might be friendly to one faction of players aren’t so
friendly to others. NPCs in the Horde lands are hostile to Alliance players, and
Alliance NPCs are hostile to Horde players. Be careful when approaching such
hostile non-player characters. The Horde and Alliance are not allies.
Enemy Type
There are many types of monsters in the World of Warcraft. They are all grouped
into several categories, or types. Sample types include beasts, undead, demons,
dragons, and humanoids. The type of monster is important because certain spells
and talents are designed to affect only certain monster types. For example, beast
slaying is an enchantment that you can place on a weapon that gives you a +2
damage bonus against all beast monsters, which include lions, raptors, bears, and
spiders.
You can tell what type the creature is by clicking on it. In the tool tip, after the
creature’s name and level, you will see its type displayed.