A mob (short for 'Monster Or Beast') is a generic term for any non-player entity whose primary purpose is to be killed for experience, quest objective, or loot. They tend to be aggressive to all players.
When a monster dies, the killer is determined by most damage dealt instead of the last hit. If you're at a party, your damages add up. Killers gain experience, quest objective, and 30 seconds of protection on item drops if any.
Monsters can cast skills just like players. Basic monsters usually have only 1 skill or no skill at all, while boss monsters have more than 5 skills.
Boss type monsters are the hardest to kill. A common characteristic for all bosses is that they have a unique name and appear once at a time. Dungeons usually have a boss in the last room that you must defeat to complete the dungeon.