Why are there two Cordova www folders?

I'm a beginner in PhoneGap and I'm trying to understand something about the www folders. I created a project with only an iOS platform.

I saw that there are two www folders:

• www in the root folder of the project
• www in the platform/ios/ folder

How do I use both of them? Am I supposed to only use only one of them?
What about sharing the HTML/JS/CSS code with other platforms?