iOS

iOS (previously iPhone OS) is a mobile operating system developed by Apple Inc. and distributed exclusively for Apple hardware.

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). As of October 2013, Apple's App Store contained more than 1 million iOS applications, 500,000 of which were optimized for iPad. These apps have collectively been downloaded more than 60 billion times. It had a 21% share of the smartphone mobile operating system units shipped in the fourth quarter of 2012, behind Google's Android. By the middle of 2012, there were 410 million devices activated. According to the special media event held by Apple on September 12, 2012, 400 million devices had been sold by June 2012.

                                                                iPhone 7.1 Homescreen

The user interface of iOS is based on the concept of direct manipulation, using multi-touch gestures. Interface control elements consist of sliders, switches, and buttons. Interaction with the OS includes gestures such as swipe, tap, pinch, and reverse pinch, all of which have specific definitions within the context of the iOS operating system and its multi-touch interface. Internal accelerometers are used by some applications to respond to shaking the device (one common result is the undo command) or rotating it in three dimensions (one common result is switching from portrait to landscape mode).

iOS shares with OS X some frameworks such as Core Foundation and Foundation; however, its UI toolkit is Cocoa Touch rather than OS X's Cocoa, so that it provides the UIKit framework rather than the AppKit framework. It is therefore not compatible with OS X for applications. Also while iOS also shares the Darwin foundation with OS X, Unix-like shell access is not available for users and restricted for apps, making iOS not fully Unix-compatible either.

Major versions of iOS are released annually. The current release, iOS 7, was released on September 18, 2013. In iOS, there are four abstraction layers: the Core OS layer, the Core Services layer, the Media layer, and the Cocoa Touch layer. The current version of the operating system (iOS 7.1), dedicates 1–1.5 GB of the device's flash memory for the system partition, using roughly 800 MB of that partition (varying by model) for iOS itself. It runs on the iPhone 4 and later, 2nd-generation iPad and later, all models of the iPad Mini, and the 5th-generation iPod Touch.

Source :iOS

Sensors For Mobile Phone

Modern mobile phones come with a variety of sensors that automate or easy many of our daily tasks. This field takes into account the presence of an accelerometer, a gyroscope, a compass, and a barometer.

Accelerometer and gyroscope

Accelerometers in mobile phones are used to detect the orientation of the phone. The gyroscope, or gyro for short, adds an additional dimension to the information supplied by the accelerometer by tracking rotation or twist.

An accelerometer measures linear acceleration of movement, while a gyro on the other hand measures the angular rotational velocity. Both sensors measure rate of change; they just measure the rate of change for different things.

In practice, that means that an accelerometer will measure the directional movement of a device but will not be able to resolve its lateral orientation or tilt during that movement accurately unless a gyro is there to fill in that info.

With an accelerometer you can either get a really "noisy" info output that is responsive, or you can get a "clean" output that's sluggish. But when you combine the 3-axis accelerometer with a 3-axis gyro, you get an output that is both clean and responsive in the same time."

Digital compass

The digital compass that's usually based on a sensor called magnetometer provides mobile phones with a simple orientation in relation to the Earth's magnetic field. As a result, your phone always knows which way is North so it can auto rotate your digital maps depending on your physical orientation.

Barometer

And finally, you may see a device sporting a barometer in its specs sheet. Contrary to what you may suggest, it has nothing to do with weather. Instead, the barometer is there to help the GPS chip inside the device get a faster lock by instantly delivering altitude data. 

Source : GsmArena