Cupcake, Gingerbread, Floyo, Android 2.3, Android 3.1, oh man, with these different names, it can be so confusing to figure out what OS you are running. Luckily, there is an easy and convenient way to know what OS version your device is running. Following are the steps(We take Samsung Galaxy S4 as an example):
Scroll to "about phone"
Look for line which says "Android Version"