MOB is a term coined way back in the old days of early games. It is actually shorthand for mobile, meaning bad guys who move around. There are several bad guy MOBs or monsters in Minecraft. They spawn or are born in dark places (like caves) or from MOB spawning cages that are located deep in hidden caves. When you see the sun setting or hear the sound of the evening music, better head home or be prepared to encouonter some monsters. They will also be found in dark places like caves. If you're going mining, you might want to bring a friend and some light, fire, and water resources.
This is the Minecraft wiki entry on Creepers . They are about as tall as your avatar and blotchy green, as if camouflaged. Creepers can be extremely destructive to you and your property or the surrounding area. If you attack them you'd better kill them fast. If you hear their signature hissssssssing sound, run, they are about to explode. While, like most MOBs they don't like light, they will not burn up in the daytime sunlight. However they are afraid of ocelots and cats. You can get a cat by taming an ocelot.
Spiders will attack you if they are in a dark or dim area. Otherwise they'll only attack you if you attack them. They are one MOB that can climb, and they often hang out in trees. Don't forget to look up! They make a slurpy sound when they are near. They have beady red eyes and grey-black bodies. Spiders also do not burn up in the daytime sunlight. Spiders drop string, which is needed for making yourself a bow or a fishing pole.
Skeletons are very dangerous because they can attack from afar. They use a bow and arrow to shoot at you. You can tell they're near when you hear the sound of rattling bones, or if an arrow comes whizzing by. They WILL burn up in the daytime sunlight, but only if htey're in direct sunlight. They can survive in the shade of a tree or cliff overhang. When killed, they drop arrows which you can harvest for your own use. They drop something useful if you're taming a Wolf.
Zombies still look a bit human, but they're dead, or undead, so they're green. They make a moaning groaning sound, and of course, walk with their arms outstretched. They WILL burn up in the sunlight, like skeletons. But also like spiders, they are safe if in a shady area or in the water (rain counts as water). They drop something useful if you're desperately hungry
The Enderman is a weird MOB. He is tall and skinny and all black except for his purple eyes. He is often seen carrying a block. Endermen will leave you alone UNLESS you look at them. Once you look at an Enderman and he looks directly back at you, he will pursue you with his ninja-style moves. He can teleport to you and attack from the side or behind. Sunight will not damange an enderman. An odd and interesting fact: besides killing an enderman with the usual sword or shovel, you can damage him with water. He hates water, even rain. Run into a lake if an enderman is chasing you. Endermen can be annoying because they will randomly pick up and move building blocks, even from structures like your house. As Dr Who would say, if you're staring at an enderman, don't blink.