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?
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.