What does the tree mean when generating a world?

When you generate a new world in Terraria (in game not through the console) an icon of a tree is shown. The same icon will be shown when selecting the world from the list:

What are all the different possibilities and what do they mean?

Solution 1:

The tree indicates what is on the world.

You can have either Purple/Green leaves which means that the world will have Corruption, or you can have Red/Green leaves which means that the world will have Crimson.

Also after defeating the Wall of Flesh (and entering hard mode) a yellow streak will appear on the tree indicating that the world has hallow in it.

In the following image, the first world has corruption and hallow, the second is a corruption world pre-hardmode, and the third image is a crimson world pre hardmode.