Mobs

Mobs in Minecraft come in three major subsets:

Passive

Hostile

And neutral

Hostile Mobs
Hostile mobs are mobs that try to attack you even if you don't hurt them. Wolves would go under this category, but only if it has become hostile towards whoever hit it first. Most of them are nocturnal. They spawn at night. This includes Zombies, Skeletons, Creepers, and Endermen (are hostile when provoked).

Zombies are similar in design to the default "Steve" skin, but is green with its arms sticking out. They can spawn with or without armor and with either an Iron Shovel or Iron Sword. If they spawn with a helmet on, then this will make them more of a threat. They will not burn.

Skeletons spawn with a bow and rarely armor or an enchanted bow. They are really annoying if you are building high up or building a bridge high up in the air. They can shoot fairly well, better than that of a beginner's in my opinion. They can pick up any headgear, including pumpkins and helmets.

What makes Minecraft so famous? Probably the Creepers in it. If you find them, either run away or shoot them with a bow from a distance. They will start to hiss and expand. Then, after about 2 seconds, they blow up everything within a 3-5 block radius, including themselves. Pure evil!

Endermen. Tall, black, and scary. They will teleport around upon being provoked, and will keep teleporting to you until they die, or lose interest in you. They are EXTREMELY valuable to complete the main goal of Minecraft:

Passive Mobs
These will not attempt to attack at all. most of them only run (or fly) around like crazy when hit. some passive mobs include sheep, cows, and pigs

Neutral Mobs
these mobs will be passive unless you hit them, then they become hostile. there are not many Neutral mobs some include enderman, zombie pigman, and wolves.