Android VS iOS
According to the latest Gartner data, in the 4th quarter of 2013, Android OS based mobile phones accounted for 78.4% of total smartphone shipment, while iOS accounted for 15.6%, it is followed Windows phone at 3.2% and RIM at 1.9%.
Android has seen massive growth in the last four years. In 2009, Android accounted for only 3.9%, now it account for nearly 80% of smartphone shipment. The reason for this massive growth is that Android is free and open source (FOSS) operating system, so it is free to be distributed and modified by manufacturers. Android is under the leadership of Google, and has Google products built in, almost everyone use some form of Google product, so the Android OS is easily trusted by users. Android's rise is mainly at the expense of RIM or BlackBerry and Symbian OS.

-Ming Ren