Mobs are living and moving game entities. The name for them is short from mobile and was carried over from the genre of old MUD games.
Mobs spawn in various ways, depending on the light level and other criteria. Many types of mobs often spawn in groups of the same species. Mobs cannot spawn on non-opaque blocks or in water (except for aquatic mobs), in lava or on half blocks like slabs. Exceptions are mob spawners.
Mobs can also be spawned via spawn eggs in creative mode or via the
Mobs have general behavior properties akin to players, as they are subject to physics and can be hurt by many damage sources that would hurt a player. Some mobs have immunity, however.