In a nutshell Amazon MP3 made a web app that allows them to bypass Apple’s terms and conditions for iDevices. Meaning that there is not an app that must be present in the Apple App Store. By doing so they avoid the whopping 30% commission made on every purchase.
A web app pretty much gives you a simular experience to a normal app downloaded from an app store. Amazon MP3 will be amping up their cloud service in order to use the web app. The new service will allow you to enjoy your music from virtually anywhere you have an internet/data connection. When purchasing music you will be able to listen to a 30 second preview prior to buying. Difference is that the music will not be downloaded to the device, rather it is stored in the cloud. There is an Amazon Cloud Player APP available for iDevices and desktops.