This page is devoted to the things that will kill you in MineCraft - hostile mobs.
Zombie - An undead slave, this mob will attack you on sight, causing medium damage. He moves slowly, and can be taken out easily witha sword, and even more easily with a bow. He has medium health. He spawns in dark areas and at night. He sometimes drops feathers on death. He burns and dies in direct sunlight.
Skeleton - A walking dead, he will attack on sight with an infinite supply of arrows. He has medium health, and can be a deadly opponent, and overpowering in groups. It is best to use a bow on this foe. He causes medium damage at range. He drops arrows and/or bones on death. He burns and dies in direct sunlight.
Spider - A weaker foe, the spider is passive unless attacked during the day, but is hostile at night. The spider can jump attack, and also climb walls. He does medium damage and is easily killed with a bow or sword. Deadly in groups. Spiders sometimes drop string, used for bows and fishing rods, on death.
Creeper - The most deadly foe in MineCraft, the creeper destroyed all. He is silent until right next to you, and is hostile. When he is within range of you, he will explode, causing high damage, deadly unless combated with a bow. He destroys blocks, and can destroy your house. He has medium health, and sometimes drops sulphur on death, which can be used to create TNT. Many players refer to him a an 'exploding green penis'.
Slime - Only spawns in deep caves underground, the slime causes medium damage, and can split into smaller versions of itself. It comes in different sizes, and drops slimeballs on death, which are used to create sticky pistons.
Zombie Pigman - Only spawns in the Nether normally, this mob is passive until attacked. When attacked, every zombie pigman near it will attack you. This makes fighting zp very hard. They have medium health and do high damage to you. They can drop cooked pork chops and/or golden swords on death.
Ghast - Ghasts are the only flying mob in minecraft. They are large, and spawn in the nether. There is a small chance of them spawning near a portal in the normal world, however. Ghasts have little health, and shhot fireballs that explode at the player. It is best to use a bow against this mob, but you can hit the fireballs back at the mob. Fireballs can also disable a portal back to the real world.