Magento theme designing is complicated unlike other cms wordpress. If we look into the magento theme structure we will notice that it contains two main folders app and skin.

The two primary elements of Magento Theme are :

•     Layout elements: It contains the files which renders the website pages
•     Design elements: It contains the files(like Images, CSS etc) which gives the page's visual.

The layout files are all resides in the design folder in app, 
where else all design element files are in skin folder.

Layout Elements 

if we look into design folder in app we will see three main subfolders base, default and rwd .These are the different design packages.
lets work on the rwd package. the rwd folder contains three subfolders 

• blank, 
• default, 
• iphone,  
• mordern

If we look in any of these folders they contains subfolders:

• layout, 
• locale, 
• template 

Layout : This includes the xml files to control the basic block structure of pages.
Template: This includes the PHTML (PHP + HTML) files  that control the page structure.
Locale: It has the files for translation in different languages for magento.

Design Element

As the design elements are located in the skin folder it also contains the frontend folder in which it has subfolders like the app => design contains.

• base, 
• default, 
• rwd 

Same way the subfolders of them are also same.
if we look in the default sub folder it has subfolders:

• css, 
• images, 
• js 

These folders contains the design elements here.