They're a great source of entertainment, whether you're killing things for food, fun or material farming. They're not so fun, however, when they're shooting projectiles at you, walking off with blocks from your painstakingly crafted monuments or blowing you up.
Which are the ones to worry about? There are a number of different types of mobs and enemies in the game.
Passive mobs aren't bothered by your presence. They won't attack you on sight, and they probably won't even get aggressive if you hit them. These mobs are a good source for food and some give low amounts of XP, giving you a small head-start on developing some bad-ass enchantments.
- Bats: They spawn in underground caves, and fly around in low light conditions. They do not drop anything when they die.
- Sheep: Drop wool and/or mutton on death. Will get into your crops and have a munch if you're not careful. Can be bred. They drop 1 wool block on death, but if they are sheared you can get up to 3. Lambs can't be sheared for wool.
- Cows/Mooshrooms: Drop uncooked beef and/or leather on death. Mooshrooms will give you mushroom stew if you use a bowl on it. Can be bred. Shearing a Mooshroom will turn it into a regular cow.
- Horses: Can be tamed, and are mountable and rideable with a saddle. Can be bred. If you breed a horse with a donkey, you can get a mule.
- Chickens: Drops uncooked chicken and/or feathers on death. Can be bred. Regularly lays eggs, and eggs have a chance to spawn baby chickens when thrown.
- Rabbits: Their skin colors depend on the biome the spawn in. They will approach the player if they are holding a carrot or a dandelion. They drop raw rabbit and rabbit hide on death.
- Villagers: You can trade with villagers for precious items, or you can evict them and steal their houses, it's up to you. The color of their robe depicts their profession. They can breed with other villagers (as long as their houses have doors, even NPC's need privacy) and make even uglier human Squidward hybrids.
- Squid: They spawn in deep water in any biome. They drop ink sacs on death and are completely harmless even if you're finding their 8-armed dancing rather terrifying. The good news is they suffocate on land, the bad news is, you can't lure them there...
- Ocelot: Ocelots spawn on grass or leaves. Feeding them an uncooked fish has a 2/3 chance of taming them. They will follow the player when tame. They can sit on objects, so taming a pretty one could make a nice bed decoration. (Cats are friends, not furniture)
- Donkey: Donkeys spawn in the same conditions as horses, and will usually make a small percent of a horse herd. Donkey's can't wear horse armor, but they can be equipped with chests.
Neutral mobs will ignore you for a time, but some of them may attack you once a certain condition has been fulfilled.
- Polar bears: Polar bears are neutral, and their cubs are passive. Polar bears become hostile if you get too close to their cubs or the cub is attacked.
- Spider/Cave Spiders: Spiders are considered passive when there is a good enough light source. They will become hostile when night falls or if you're in a dark environment.
- Endermen: Endermen are passive and relatively rare. They spawn in small groups. They will remain passive unless attacked or you look them in the eyes. Endermen often take natural blocks and move them around. They don't like water or sunlight too much and will teleport away if it is raining.
- Wolves: Wolves spawn in forest and taiga biomes. If you have a bone (or ten), you can tame the wolf and he will follow you and attack anything that attacks the player. When the wolves are wild, they remain passive to the player unless attacked. When a wolf is attacked, its eyes turn red and any other wolves in the vicinity will join in the fight against you. If your tamed wolf takes damage, you can see its health by the level at which it holds its tail. You can feed it to heal it. Skeletons avoid wolves, so they're good to have around for protection.
- Zombie Pigmen: Zombie Pigmen live in the Nether. They are generally passive until you attack one, then all of the Zombie Pigmen in the vicinity will converge upon you. Squad goals.
Hostile mobs are the enemies you don't want to get ganged up on by. They will attack on sight, and do their damn best to kill you. The hostile mobs are often the ones that hold the best materials for potion making and crafting, so the benefits and the risks can cancel each other out.
- Blazes: Blazes are found in the Nether fortresses at low light levels. They shoot fireballs at the player, and if there are other Blazes nearby, it will alert them to your presence. They drop Blaze rods which can be used in potion-making. They're weak to water, and snowballs are effective against them as weapons. If it isn't already obvious, Blazes are immune to fire and lava.
- Chicken Jockeys: As the name suggests, Chicken Jockeys are tiny skeleton mobs that ride chickens. They are like Spider Jockeys on a much smaller scale.
- Creepers: Anyone who has ever played Minecraft has already come across these explosive green phalluses. Creepers are reasonably slow, but on sight will walk towards you hissing, until they blow up in your face. They will not only destroy anything they are near, but their destructive power can leave you with a lot to rebuild. Best to take them out from a distance before they get to the explodey stage.
- Elder guardians: resembling a square puffer fish, Elder Guardians spawn within ocean monuments. They shoot a laser at players and can track them underwater, doing huge damage and afflicting them with Mining Fatigue. They drop prismarine shards which can be used to create sea lanterns.
- Endermites: Endermites are spawned when an Ender Pearl is thrown. Endermen are hostile to them, and will attack them if a player is not around. Endermites behave like Silverfish and they're the smallest mob in Minecraft.
- Ghasts: Ghasts spawn in the Nether. They are flying enemies that will shoot ghast fireballs at you. These fireballs are incredibly damaging, so it is important to protect your Nether portal from the Nether side. Cobblestone can easily withstand the blast damage from a ghast fireball, making it an effective material for keeping your exit safe. Ghasts look like crying white octopi and will only ever attack the player by a line of sight and not other monsters.
- Guardians: These are very similar to Elder Guardians, in most ways except appearance and spawning particulars. They still spawn within ocean monuments, though are more likely to be inside them than with the open sea above them.
- Husks: Husks are similar to zombies. They behave in the same ways with a couple of subtle differences. If a zombie is spawned beneath the open sky, it has a high probability of being a Husk. However, they do not burn in the sunlight like zombies do, and they avoid water.
- Magma cubes: Magma cubes spawn anywhere in the Nether at any light level, but are more likely to spawn within the Nether Fortress. They behave like the other slimes, breaking into smaller pieces when they are attacked. They are immune to fire and lava, naturally; and their jump height is dependent on their size. There is also a possibility that on death, a Magma cream will be dropped.
- Shulkers: These mobs blend in with the purpur blocks on the islands in The End. An outer shell protects the monster within, and it will peep out every few second. Projectiles are a good way to take these blockheads out.
- Silverfish: Silverfish live within certain blocks found in strongholds and extreme hills biomes. They attack when their eggs within a block are broken, and will call to other nearby silverfish who will seek you out. It's easy to get swamped by them, so it's best to avoid engaging them at all if possible.
- Skeletons: Ahh the bane of my 'Craftin' life. Skeletons will shoot arrows at you on sight, relentlessly chase you and be a general pain in the backside. They spawn anywhere with low light levels, but the good news is they catch fire in sunlight. Take that, bone-man; here comes the dawn!
- Slimes: Gelatinous cubes that try to flop on you on sight. They break into smaller slimes as they take damage, eventually being small enough to kill outright. They spawn in very low light conditions, mostly underground and at night in swamps. They drop slime balls on death which can be used to make sticky pistons.
- Spider jockeys: There is a 1% chance of a spider spawning with a skeleton riding on its back. The AI follows the spider's lead, so the spider jockey will ignore you during the day, and attack you and chase you during the night. When killed the spider drops string and the skeleton may drop arrows or bones, as they would if killed independently.
- Strays: Strays are variants of skeletons. They spawn directly beneath the sky in icy biomes. They behave the same as skeletons, but their arrows inflict slowness. When they're killed they have a chance of dropping these slow-poison tipped arrows.
- Witches: Resembling villagers wearing pointy hats, Witches will approach you while their bodies are smoking with defensive magic. They attack using splash damage potions, and spawn in the Overworld in low light conditions. Witches can be made if a lightning bolt strikes within a small area of a villager, or they can be found in Witches huts that spawn on the map. They drop gunpowder when they die, which is great for making TNT.
- Wither skeletons: These dangerous mobs spawn within Nether fortresses in very low light levels. If they've not spotted you they'll walk along slowly, but once you're in their sights they'll sprint after you. They inflict Wither on the player if they get hit which turns your health bar black and eats at your hearts. These merchants of chaos are able to pick up and use dropped weapons and armor.
- Zombies: One of the most common mobs in the games, Zombies spawn in the Overworld during the night. They catch fire in the sun, and they drop rotten meat on death. They chase the player and do physical damage on contact. There are a few variations, such as Baby Zombies (who doesn't burn in sunlight) and Zombie villagers (who can be cured).
Boss mobs are incredibly powerful enemies, with high health and a high damage output. They spawn in very specific locations and can be re-fought.
- Ender Dragon: Ender Dragons spawn in The End, as soon as you exit the Portal. It is incredibly hostile, shooting Ender charges at the player. It has four modes of attack, where it circle the battlebround and then swoop in to shoot at you; it will charge at the player; or it will perch, during which time it is immune to arrow attacks. The Ender dragon's breath can be collected in an empty bottle.
- Wither: These hell-spawn can be created by placing soul sand in a T-shape and adding three wither skeleton skulls to the top (omg why would you?). When it activates, it grows larger and health bar doubles (seriously, you're crazy). It'll then explode, damaging you and everything around you. The wither is hostile to everything. It can regenerate health, and when it drops below half health, it becomes immune to arrows (crazy, I tell you).