Mobs
This Mob is very common, and used to get food.
Pig
You find this everywhere, even underground.
Dirt
Made from trees.
Stick
This generates in oceans, and you explore this more than anywhere.
Islands
Notch will add this in the next update.
Map
It floats, and likes to go down.
Squid
Red, hot, and is everywhere.
Netherrack
Can be cooked, and found in water.
Fish
Tool items are found in here more often than on top of the land.
Caves
They walk at night and burn in daylight.
skeleton/zombie
It floats, but doesn't touch water.
Ghast
Can't float, but is found underground.
Gravel
Colorful, and made of many items.
Dyes
Deep, and usually isn't explored much.
Oceans
This will be added officially, but won't look like it does now.
Pistons
It jumps on you and doesn't die from sunlight.
Spider
Can't be touched, can't be mined, but its everwhere.
Air
Can't be found in the Overworld, you have to dig deeper than the deepest.
Glowstone
Hard to find on land, but can be used to burn things.
Lava Springs
You can spawn them, but they wont be implemented officially yet.
Giant
It is weird, and creeps up on you.
Creeper
Hard, but not in real life.
Obsidian
Can't collect it, not even in the actual game.
Quiver
A land completely different from usual.
Nether
Everyone knows him by now.
Herobrine