Google Playstore is for any mobile phone that runs the popular Android operating System. As long as your phone runs the OS and you have a Gmail account you can purchase and download from the store.
Itunes caters for any smartphone that run the amazing IOS mobile operating system. Its only for Apple products.
The content of these 2 is the same cause apps have been designed to support different Operating systems ie Android, IOS. There is also appworld for Blackberry, Ovi Store for SYmbian and WIndows store for WIndows Operating system.