Clash of Clans is a popular iPad/iPhone/iPod/Android game created by "Supercell". It has been available internationally on the iTunes store for free since its initial v1.7 release on 2 August 2012, and on the Google Play Store since 8 October 2013. Clash of Clans is a strategy game where players can construct and expand one's village, unlock successively more powerful warriors and defenses, raid and pillage resources from other villages, create and join Clans and much, much more.
Although Clash of Clans is Free-to-Play, additional in-game currency can be purchased for real money from the Apple App Store/Google Play Store in the form of In-App Purchases. However, all features of the game are open to players who choose not to make In-App Purchases. Clash of Clans is available on iOS and Android devices!
Source: Clash of Clans Wikia, "Clash Of Clans", available under the CC-BY-SA License.