operating system

•  iOS (previously iPhone OS) is a mobile operating system developed by Apple Inc. and distributed exclusively for Apple hardware. It is the operating system that powers many of the company's iDevices.

 Originally unveiled in 2007 for the iPhone, it has been extended to support other Apple devices such as the iPod Touch (September 2007), iPad (January 2010), iPad Mini (November 2012) and second-generation Apple TV onward (September 2010).
 http://en.wikipedia.org/wiki/IOS

•  iOS 8 is the eighth major release of the iOS mobile operating system designed by Apple Inc. as the successor to iOS 7.

Many of the features and highlights of iOS 8 have seem to come from previous versions of Android, and even Windows Phone.
http://en.wikipedia.org/wiki/IOS_8

•  Chrome OS is a Linux kernel-based operating system designed by Google to work primarily with web applications

The user interface takes a minimalist approach and consists almost entirely of just the Google Chrome web browser
This means that Chrome OS is almost a pure web thin client OS
Chrome OS is built upon the open source project called Chromium OS[10] which, unlike Chrome OS, can be compiled from the downloaded source code.
Chrome OS is the commercial version installed on specific hardware from Google's manufacturing partners.
http://en.wikipedia.org/wiki/Chrome_OS

• Chromium OS is a Linux-based operating system designed by Google to work exclusively with web applications. It is the open source development version of Chrome OS.

http://en.wikipedia.org/wiki/Chromium_OS

• Android (operating system)

Android is a mobile operating system (OS) based on the Linux kernel that is currently developed by Google.
http://en.wikipedia.org/wiki/Android_%28operating_system%29