Each world in Bedrock is a floating island composed of different kinds of blocks. The surface is composed of dirt (with rocky dirt scattered throughout), a solid layer of rocky dirt underneath, and patchy grass on top. The remaining volume is stone.
An important thing to note is that the world wraps in all three dimensions. For example, if you made a long bridge out from the island, you would eventually reach the other side of the island. Or if you fell off, you would fall back onto the top.
In future versions, the surface will be populated with trees, rivers, etc. and underground there will be ores and (rarely) caves. There may also be multiple islands to explore, each having a different "theme" or biome.
As of dev_0.0.3 the maximum dimensions of the world are 512*512*512 blocks.
Structures are naturally-occurring objects that are separate from the terrain. Currently, the only structures generated are boulders.
In the future, there will be other structures such as trees and abandoned buildings.
The world generation algorithm is responsible for creating new worlds and populating them with features, such as trees and ore. Currently, this takes ~3 seconds. The world is now saved automatically when the game quits.
The algorithm starts out by generating several perlin noise maps, one for the mountains, one for the "flat" areas, one for the bottom, one for soil thickness, and one for grass density. It then fills the volume between the bottom and mountains heightmaps with stone, with a top layer of dirt and rocky dirt. Next, it fills any areas below a certain elevation up to that elevation, and applies the "flat areas" heightmap. Finally, it lays down grass based on the grass map, with the most dense areas being tall grass. In the future, more vegetation, lakes, rivers, and ores will be placed, and there might be abandoned structures both above- and underground. There might also eventually be erosion and other base terrain features such as cliffs.