Symbian OS is an operating system, designed for mobile devices, with associated libraries, user interface frameworks and reference implementations of common tools, produced by Symbian Ltd. It is a descendant of Psion's EPOC and runs exclusively on ARM processors.

Symbian is currently owned by Ericsson (15.6%), Nokia (47.9%), Panasonic (10.5%), Samsung (4.5%), Siemens AG (8.4%), and Sony Ericsson (13.1%). Whilst BenQ has acquired the mobile phone subsidiary of Siemens AG the Siemens AG stake in Symbian does not automatically pass to BenQ - this will need the approval of the Symbian Supervisory Board.

Latest version of Symbian

• Symbian OS v6.0 and v6.1. Sometimes called ER6. The first 'open' Symbian OS phone, the Nokia 9210, was released on 6.0.

• Symbian OS v7.0 and v7.0s. First shipped in 2003. This is an important Symbian release which appeared with all contemporary user interfaces including UIQ (Sony Ericsson P800, P900, P910, Motorola A925, A1000), Series 80 (Nokia 9300, 9500), Series 90 (Nokia 7710), S60 (Nokia 6600, 7310) as well as several FOMA phones in Japan.
  

• Symbian OS v8.0. First shipped in 2004, one of its advantages would have been a choice of two different kernels (EKA1 or EKA2). However, the EKA2 kernel version did not ship until SymbianOS v8.1b. The kernels behave more or less identically from user-side, but are internally very different. EKA1 was chosen by some manufacturers to maintain compatibility with old device drivers, whilst EKA2 offered advantages such as a hard real-time capability. v8.0b was deproductized in 2003.
  

• Symbian OS v8.1. Basically a cleaned-up version of 8.0, this was available in 8.1a and 8.1b versions, with EKA1 and EKA2 kernels respectively. The 8.1b version, with EKA2's single-chip phone support but no additional security layer, was popular among Japanese phone companies desiring the realtime support but not allowing open application installation.
  

• Symbian OS v9.0. This version was used for internal Symbian purposes only. It was deproductised in 2004. v9.0 marked the end of the road for EKA1. v8.1a is the final EKA1 version of SymbianOS.
  

• Symbian OS has generally maintained reasonable binary compatibility. In theory the OS was BC from ER1-ER5, then from 6.0 to 8.1b. Substantial changes were needed for 9.0, related to tools and security, but this should be a one-off event. The move from requiring ARMv4 to requiring ARMv5 did not break backwards compatibility. A Symbian developer proclaims that porting from Symbian 8.x to Symbian 9.x is a more daunting process than Symbian says. [2]

• Symbian OS v9.1 released early 2005. It includes many new security related features, particularly a controversial platform security module facilitating mandatory code signing. Symbian argues that applications and content, and therefore a developers investment, are better protected than ever, however others contend that the requirement that every application be signed (and thus approved) violates the rights of the end-user, the owner of the phone, and limits the amount of free software available. The new ARM EABI binary model means developers need to retool and the security changes mean they may have to recode. S60 3rd Edition phones have Symbian OS 9.1. Sony Ericsson is shipping the M600i based on Symbian OS 9.1 and should ship the P990 in Q3 2006. The earlier versions had a fatal defect where the phone hangs temporarily after the owner sent hundreds of SMSes. However, on 13 September 2006, Nokia releases a small program to fix this defect[3].
  

• Symbian OS v9.2 released Q1 2006. Support for Bluetooth 2.0 (was 1.2) and OMA Device Management 1.2 (was 1.1.2). S60 3rd Edition Feature Pack 1 phones have Symbian OS 9.2.
  

• Symbian OS v9.3 released on 12 July 2006. Upgrades include native support for Wifi 802.11, HSDPA, Vietnamese language support.

More information about symbian,  (  symbian official site : http://www.symbian.com )