Symbian

Symbian is a mobile operating system (OS) and computing platform designed for smartphones  and currently maintained by Accenture. The Symbian platform was created by merging and integrating software assets contributed by Nokia, NTT DoCoMo, Sony Ericsson and Symbian Ltd., including Symbian OS assets at its core, the S60 platform, and parts of the UIQ and MOAP(S) user interfaces.

Symbian Version history:
Symbian releases are styled Symbian^1, Symbian^2 etc. (vocalised as "Symbian one", "Symbian two").

• Symbian^1, being the first release, forms the basis for the platform. It incorporates Symbian OS and S60 5th Edition (which is built on Symbian OS 9.4) and thus it was not made available in open source.
• Symbian^2 was the first royalty-free version of Symbian. While portions of Symbian^2 are EPL licensed, most of the source code is under the proprietary SFL license and available only to members of the Symbian Foundation. On June 1, 2010, a number of Japanese companies including DoCoMo and Sharp announced smartphones using Symbian^2.
• Symbian^3 was announced on 15 February 2010. It was designed to be a more ‘next generation’ smartphone platform. The Symbian^3 release introduced new features like a new 2D and 3D graphics architecture, UI improvements, and support for external displays via HDMI. It has single tap menus and up to three customizable homescreens. The Symbian^3 SDK (Software Development Kit) was released September 2010.
• Eleven smartphones with the Symbian^3 operating system (or its updated Anna and Belle variants) have been released so far; the Nokia N8, Nokia C6-01, Nokia E7-00, Nokia C7-00, Nokia E6, Nokia X7, Nokia 500, Nokia 600, Nokia 603, Nokia 700, and Nokia 701.
• Symbian^4 was expected to be released in the first half of 2011. However, Nokia announced in October 2010 that Symbian^4 will not ship in a separate release. Instead, many of the UI enhancements planned for Symbian^4 will be released as updates to Symbian^3.
• Symbian Anna is an update to Symbian^3, released by Nokia in April 2011 as part of the launch of the X7 and E6 smartphones. Symbian Anna includes such improvements as a new browser, a virtual keyboard in portrait orientation, new icons and real-time homescreen scrolling. On August 18, 2011, Nokia made the Symbian Anna update available for owners of the N8, E7, C7 and C6-01 models as a software update (via OTA update and the Nokia Software Updater/Nokia Ovi Suite PC applications).
• Symbian Belle is an update to Symbian Anna. In the summer of 2011 videos showing an early leaked version of Belle running on a Nokia N8 were published on YouTube. On August 24, 2011, Nokia announced Belle officially for four new smartphones, the Nokia 600, Nokia 603, Nokia 700, and Nokia 701. They also announced that Belle would be coming to all existing Symbian^3 devices in the fourth quarter of 2011. Symbian Belle adds to the Anna improvements with a pull-down status/notification bar, deeper near field communication integration, free-form re-sizable homescreen widgets, and six homescreens instead of the previous three.
• Nokia presented the next generation of Symbian releases to a limited audience on the Nokia Developer Day in November 2011, held in Mexico. Nokia keep following the alphabetical naming convention with Symbian Carla and Symbian Donna. Symbian Carla is planned to support Dolby Surround and enhanced NFC features as well as a new browser application. Symbian Donna will be the first release to support Dual Core processor.

Source: Wikipedia