iOS (formerly iPhone OS) is Apple's mobile operating system. Originally developed for the iPhone, it has since been extended to support other Apple, Inc. devices such as the iPod Touch, iPad, and Apple TV. Apple, Inc. does not license iOS for installation on non-Apple hardware. iOS is derived from Mac OS X, with which it shares the Darwin foundation, and is therefore a Unix-like operating system by nature.
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. The response to user input is immediate and provides a fluid interface. 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).
Apple released iOS 4 on June 21, 2010, three days before the iPhone 4, in an effort to reduce the strain on Apple's servers. iOS 4 was the first version of the OS to be a free upgrade on the iPod touch; Apple had charged $4.99 for earlier upgrades. Apple previously announced that iPad users with 3.x software would receive a free upgrade to the next major (4.x) release.
iOS 4.0.1 included a fix to the reception signal strength indicator(s). It was released on July 15, 2010, the day before Apple hosted a press conference to discuss its response to the widely publicized iPhone antenna issues. Apple also released iOS 3.2.1 for the iPad which tweaked the tablet's Wi-Fi connectivity, video playback, and copy-and-paste for PDF attachments, among other updates.
iOS 4.1 for the iPhone and iPod touch was released on September 8, 2010; the update fixed some bugs reported by users, improved battery life and added several new features: Game Center, which allows gamers to play multiplayer games online, upload high scores and unlock achievements; The ability to toggle spell checking on and off; HDR photography (iPhone 4 and 4S only); Ping, a social music network and discovery tool. iOS 4.1 also updated carrier settings which (at that time in the United States, AT&T 8.0 only). At the same time, TV show rentals became available on the iTunes US Store.
iOS 4.2.5 was released as a demo version for the CDMA version of the iPhone 4. This iPhone 4 variant was released for Verizon Wireless customers in the US on February 10, 2011, but pre-ordering was available for qualified Verizon Wireless Customers on February 3. The version released with the phone, 4.2.6, has some slight UI changes specifically for the CDMA version of the phone, including a "Personal Hotspot" switch in the Settings app, a service which has to be activated by the carrier in order for the feature to be usable. This became available for purchase from Verizon as a separate plan, as is currently possible on other smartphones.
iOS 5.0 and its features were announced on June 6 at the WWDC 2011 keynote address. The update was released at 6pm GMT on October 12, 2011. iOS 5 introduced the iCloud service and the Notification Center, as well as improvements to native apps such as Camera. The operating system also features new applications, such as the "Reminders" app and "Newsstand", an application that lets you buy newspapers and magazines. Newsstand is the first native application to run on the homescreen. "iMessage" is an application that allows iPod touch, iPhone, and iPad users to communicate, much like a chat service only used between these devices. The iMessage feature has been integrated into the "Messages" application on the iPhone. The iPod application on the iPhone and iPad has now been split into the Music and Video application, just as it had been on the iPod touch. it also added updated security and bug fixes. iOS 5 also brings long-awaited Hindi virtual keyboard. According to Apple, the new OS has over 200 new features. The launch of the iOS5 update on 12 October, 2011, (including iOS 5.0.1 released on November 10) led many users to report a major wifi bug casuing the device to lose wifi access. As of 26 December, 2011, this problem remained unresolved.