Monsters or "mobs" as they are known, can be either Hostile or Passive (friendly and neutral).
This can be determined by the colour of their health bar (turned on by pressing V).
Friendly Mobs with a blue health bar cannot be attacked and will assist player (All mobs in towns, certain NPCs or NPC groups outside of towns)
Neutral Mobs with a green health bar will ignore player unless attacked (Many tameable mobs outside of towns)
Hostile Mobs with a red health bar will attack any player or friendly mob they see (Monsters and certain NPCs or NPC groups outside of towns)
A mobs level, compared to YOU, is indicated by the colour of their name.
MOBNAME = White is below your level
MOBNAME = Blue is equal your level
MOBNAME = Green is just above your level
MOBNAME = Orange will be a challenge
MOBNAME = Red will be VERY difficult
There are occasions where you may see a + and a number at the end of the mobs name. The higher the number the harder it will be.
Example: Blue text (equal level) with a +2 means it will be more of a challenge to kill. If it's red text (very hard) with a number, it's best to stay away unless you are with a group of experienced players.
It seems that the "+x" identifies the tier of dropping. (i.e. +4 enemies drop a lot of yellow (legendary) items)
There are 3 sections for Monsters, there are:
Boss / Rare Mobs
Hostile Mobs
Passive Mobs
Use the dropdown menu and select which Mob Type you want to know more about...
As there is still very little information on each Mob type, these sections are very sparse.
I will be filling the tables up as I find out more information about each one and its characteristics.