Minecraft has several different types of enemies and neutral creatures. Some of these are useful, some are useless, and one will destroy half of that house you just spent hours building.
These creatures are commonly known as mobs. Some mobs will only spawn in darkness. Here's a short list of the hostile creatures and what you can do with them. The pictures are from the Minecraft wiki. There's much
more detailed explanations over at the wiki.
Zombies
Zombies will only spawn in the dark. This means at night, they'll come out, and if you're not protected inside a house, they'll make a bee-line for you. You are just a tasty snack to them. Kill them and they drop feathers. Zombies will burst into flames on sight of the sun. Arguably this is even worse, because the only thing worse than a zombie, is a flaming zombie...