Android is a Open-source Linux based Operating System designed to work on any device. Google purchased Android OS from ANDROID Inc. in 2005. That was the year when Android’s first version is developed.
Because ANDROID is an Open Source Software there are many customized versions of android available like CyanogenMod, MIUI etc.
Android Development cycle is short as compared to other operating systems like iOS or Windows Phone. This is the reason we see new updates in about every 6 months.