Features of iOS

Home screen
The home screen (rendered by "SpringBoard") displays application icons and a dock at the bottom of the screen where users can pin their most frequently used apps. The home screen appears whenever the user switches on the device or presses the "Home" button (a physical button on the device). The screen has a status bar across the top to display data, such as time, battery level, and signal strength. The rest of the screen is devoted to the current application.
Since iOS version 3.0, a Spotlight Search function has been available on the left page of the home screen page allowing users to search through media, applications, emails, contacts and similar files.


Folders
With iOS 4 came the introduction of a simple folder system. When applications are in "jiggle mode", any two can be dragged on top of each other to create a folder, and from then on, more apps can be added to the folder using the same procedure, up to 12 on iPhone and iPod touch and 20 on iPad. A title for the folder is automatically selected by the type of applications inside, but the name can also be edited by the user.

 Notification Center
In the iOS 5 update, the notifications feature has been completely redesigned. Notifications now collate in a window which can be dragged down from the top of the screen, much like Google's Android Operating System. If a user touches a received notification, he/she will go to the application that sent the notification.

 Included applications
The iOS home screen contains these default "apps". Some of these applications are hidden by default and accessed by the user through the Settings app or another method—for instance, Nike+iPod is activated through the Settings app.

The iPod touch retains the same applications that are present by default on the iPhone, with the exception of the Phone, Messages, Compass and Camera (before the 4th generation) apps. The "iPod" App previously present on the iPhone was split into two apps with iOS 5, Music and Videos, as it always has been on the iPod touch. The bottom row of applications is also used to delineate the iPod touch's main purposes: Music, Videos, Safari, and App Store (Dock Layout was changed in 3.1 Update). For the 4th Generation iPod touch, it includes FaceTime and Camera, and the dock layout had changed to Music, Mail, Safari, Video. As of iOS 5.0, "iMessage" will be available on all iOS devices running iOS 5. iMessage is effectively a version of the iPhone Messages app that sends free text or multimedia messages to other iOS devices (similar to BlackBerry Messenger).

The iPad comes with the same applications as the iPod touch excluding Stocks, Weather, Clock, Calculator, and the Nike + iPod app. Separate music and video apps are provided, as on the iPod touch, although (as on the iPhone) the music app is named "iPod". Although, that was changed in iOS 5 to "Music", to match the other devices in the family. In iOS 5, the iPod app will be replaced by Music and Video apps on all devices. Most of the default applications are completely rewritten to take advantage of the iPad's larger display. The default dock layout includes Safari, Mail, Photos and Music.

Multitasking
Before iOS 4, multitasking was limited to a selection of the applications Apple included on the devices.Starting with iOS 4, on 3rd-generation and newer iOS devices, multitasking is supported through seven background APIs:

    Background audio - application continues to run in the background as long as it is playing audio or video content
    Voice over IP - application is suspended when a phone call is not in progress
    Background location - application is notified of location changes
    Push notifications
    Local notifications - application schedules local notifications to be delivered at a predetermined time
    Task completion - application asks the system for extra time to complete a given task
    Fast app switching - application does not execute any code and may be removed from memory at any time

Switching applications
Double-clicking the home button activates the application switcher. A scrollable dock-like interface appears from the bottom, moving the contents of the screen up. Choosing an icon switches to an application. To the far left are icons which function as music controls, a rotation lock, and on iOS 4.2 and above, a volume controller. Holding the icons briefly makes them "jiggle" (similarly to the homescreen) and allows the user to force quit the applications by simply tapping the red minus circle that appears at the corner of the app's icon.

Game Center
Game Center is an online multiplayer "social gaming network"released by Apple. It allows users to "invite friends to play a game, start a multiplayer game through matchmaking, track their achievements, and compare their high scores on a leader board." iOS 5 and above adds support for profile photos.

Game Center was announced during an iOS 4 preview event hosted by Apple on April 8, 2010. A preview was released to registered Apple developers in August. It was released on September 8, 2010 with iOS 4.1 on iPhone 4, iPhone 3GS and iPod touch 2nd generation through 4th generation. Game Center made its public debut on the iPad with iOS 4.2.1. There will be no support for the iPhone 3G and original iPhone. However, Game Center is unofficially available on the iPhone 3G via a hack.

Development
The applications must be written and compiled specifically for iOS and the ARM architecture. The Safari web browser supports web applications as with other web browsers. Authorized third-party native applications are available for devices running iOS 2.0 and later through Apple's App Store.

SDK
On October 17, 2007, in an open letter posted to Apple's "Hot News" weblog, Steve Jobs announced that a software development kit (SDK) would be made available to third-party developers in February 2008.The SDK was released on March 6, 2008, and allows developers to make applications for the iPhone and iPod touch, as well as test them in an "iPhone simulator". However, loading an application onto the devices is only possible after paying an iPhone Developer Program fee.
As of January 7, 2012, the fees to join the respective programs for iOS and OSX were stated at $99.00 per app.Since the release of Xcode 3.1, Xcode is the development environment for the iOS SDK. iPhone applications, like iOS and Mac OS X, are written in Objective-C.

Developers are able to set any price above a set minimum for their applications to be distributed through the App Store, of which Apple will take 30% of the revenue (the other 70% goes to the developer). Alternately, they may opt to release the application for free and need not pay any costs to release or distribute the application except for the membership fee.

 Jailbreaking
Ever since its initial release, iOS has been subject to a variety of different hacks centered around adding functionality not allowed by Apple. Prior to the 2008 debut of the native iOS App Store, the primary motive for jailbreaking was to install third-party native applications, which was not allowed by Apple at the time.Apple has claimed that it will not release iOS software updates designed specifically to break these tools (other than applications that perform SIM unlocking); however, with each subsequent iOS update, previously un-patched jailbreak exploits are usually patched.Ever since the arrival of Apple's native iOS App Store, and—along with it—third-party applications, the general motives for jailbreaking have changed. People now jailbreak for many different reasons, including pirating App Store applications, gaining filesystem access, installing custom device themes, and modifying the device SpringBoard.In 2010, the EFF successfully convinced the U.S. Copyright Office to reject Apple's claim that jailbreaking is in violation of the Digital Millennium Copyright Act and declare that iOS jailbreaking is legal in the United States.

Digital rights management
The closed and proprietary nature of iOS has garnered criticism, particularly by digital rights advocates such as the Electronic Frontier Foundation, computer engineer and activist Brewster Kahle, Internet-law specialist Jonathan Zittrain, and the Free Software Foundation who protested the iPad's introductory event and have targeted the iPad with their "Defective by Design" campaign.Competitor Microsoft, via a PR spokesman, has also criticized Apple's control over its platform.

At issue are restrictions imposed by the design of iOS, namely DRM intended to lock purchased media to Apple's platform, the development model (requiring a yearly subscription to distribute apps developed for the iOS), the centralized approval process for apps, as well as Apple's general control and lockdown of the platform itself. Particularly at issue is the ability for Apple to remotely disable or delete apps at will.

Some in the tech community have expressed concern that the locked-down iOS represents a growing trend in Apple's approach to computing, particularly Apple's shift away from machines that hobbyists can "tinker with" and note the potential for such restrictions to stifle software innovation. However, there are some outside of Apple who have voiced support for the iOS closed model. Facebook developer Joe Hewitt, who had previously protested against Apple's control over its hardware as a "horrible precedent", has subsequently argued the locked apps in the iPad are akin to web applications and provide added security.