Four worlds

The four worlds is a way of looking at or interpreting the Tree of Life

The four worlds are called
 * 1) Atziluth - The world of Archetypes, likened to Fire
 * 2) Briah - The Creative world, Water
 * 3) Yetzirah - The Formative world of Air
 * 4) Assiah - The Active world of Earth

One system divides the Tree of Life into 4 worlds, and another system places 4 Tree of Life upon each other to represent these worlds.

Creation Of The Universe
One way to interpret these 4 worlds is as the way the universe was created.

First God would operate in the world of Archetypes or Emanations, this is the world called Atziluth. After all things are arranged in that world the creative process begins in the world of Briah. Then things start to get shape, the basis for the universe is formed in the world of Yetzirah. Then the last step forms the active world as we experience it, and the creation is completed in the world of Assiah

Applying the Philosophy
This interpretation can also be applied to various mundane tasks, say you have to build something, then you to go trough these 4 worlds to give birth to your own creation. If perhaps you are unable to create a certain thing in your life, you may be overlooking one of these 4 worlds. By careful observing you could find out which world, and study that world, to see what is missing.

an example: Perhaps you have many ideas, but don't get past the creative world. Or perhaps you are always working in the active world, without ideas so not able to build anything useful.