A mob is a DNPC (Dynamic Non Player Character) that may be attacked and killed for experience and items. Mobs spawn in set areas, and have a very short respawn time. Killing mobs is the main way in which players gain experience points and increase their level. The experience gained from a mob is a function of the mob's level and the character's. Each mob has a set of items that they may drop including but not limited to: gold, relics, elixirs, Blood Essence, and weapons.
The strength of a mob can be estimated by the colour of its name. If the name is dark blue, then they are not a threat, but if the name is red, then they are very dangerous. The colour depends on your level difference with the mob. Once a mob is dark blue, you can no longer receive experience or items for killing it.
Mobs also have different levels of aggresiveness (Aggro), ranging from pasive to social aggresive, the last one meaning that if you attack any other mob in its vicinity, the "social" mob will join the fight and attack you. Beware of mob behaviour, because attacking a dark blue social mob may make others to join, and get killed even by mobs of level way lower than yours.