Mobs are creatures in Minecraft. They are affected by gravity (unless they fly) and come in three different groups: Passive, Neutral, and Hostile. All dimensions have mobs naturally spawning in them. Utility mobs are created by the player to defend them and bosses are for the player to fight in hopes of getting great loot. Generally, here is what the three groups mean:
Passive - cows, chickens, dogs, sheep etc - these mobs will never harm the player, not even in retaliation
Neutral - wolves, Endermen, spiders (during the day) etc - these mobs will not actively hunt down the player, but will attack them if they are attacked (or offended, in the case of the Endermen)
Hostile - Creepers, zombies etc - these mobs will attack on sight, with no provocation, and have no regard for their own safety in doing so.