Mobile object, or "MOB", in Guild Wars typically refers to a monster, or a non-player hostile creature.
Generally, Mob is the slang term given to any non-player character (NPC) or combatant in a video game. The term has been in use since the early days of online gaming, in the MUD community, then used in the more general sense that any NPC was a mob.
Note that many players will use the word "mob" to refer to a group of enemies gathered at a certain point (following the basic meaning of the English word "mob"). The two meanings can often be interchanged seamlessly in a sentence with one person using "mob" to indicate one meaning but another person understanding the sentence clearly while thinking of the other meaning. An example would be: Go attack that mob over there.
For a list of "Mobs", see: Category:Bestiary.