The zombie is one of the most common enemy mobs you will run into. Zombies walk very slowly and attack when you are close to it. They can wear armor and pick up weapons or any other items. Zombies do not drop their items until you kill them. They have a low chance of dropping potatoes and iron as well as their armor. Zombies are most likely to drop rotten flesh. If this mob steps into sunlight unless it wears a helmet it will be set on fire.
The creeper is one of Minecraft's most dangerous mobs to come across. If you come close to it the creeper will try to explode possibly killing you in the process as well as its surroundings. The creeper has an even more dangerous form as well it is called the charged creeper. A charged creeper only appears when struck by lightning. It does much more damage and destroys even more of its surroundings. I would suggest you bring a shield with you to block the explosion. The creeper drops gunpowder which can be used for fireworks or tnt.
The skeleton is an annoying mob to come across. It has a bow and shoots you from a distance. Skeletons can drop bones which can be turned into bone meal to grow crops. They also have a chance to drop their bow, but it has low durability so it will break after using it a few times. They can drop arrows which you can use with your bow. When this mob encounters sunlight it will be set on fire unless it wears a helmet.