Here's a summary and brief details on each release.
Symbian^2
Symbian^2 is the first full release under the Symbian Foundation. It is the equivalent to S60 5th Edition Feature Pack 1 (S60 5.1). It is the first release for which the source code is (will be) available via the Symbian Foundation website. Some of the code is being released under EPL, although the majority is still under SFL.
In Symbian ^2 the customisable, widget-based homescreen, as seen in the N97, becomes a formal part of the platform. It will also support multiple form factors, screen resolutions (including VGA) and input methods.
Symbian^2 will be functionally complete by week 19, 2009 (next week) and will be hardened by week 51. This means it will be available in devices at the very end of the year - or early next year.
The Foundation blog notes that "Symbian^2 is particularly suitable for device manufacturers who have already been working with previous versions of Symbian OS technology. Device manufacturers who are relatively new to the Symbian world are expected to use Symbian^3 in their first commercial shipments". This means Symbian^2 will likely be used by Nokia, Samsung and Sony Ericsson in a whole range of devices that are due in the market in 2010.
Symbian^3
Symbian^3, in many ways, is the first Symbian Foundation release that fulfills the vision of the Symbian Foundation. Most, if not all, of the code will be available under EPL. This release will see Screenplay graphics, Open acceleration and improved multi screen support.
Symbian^3 incorporates the technologies that Symbian have previously promoted as Freeway and Screenplay. These consist of a 'high performance communications architecture' (including true seamless handover between different bearers) and 'support for seamless composition of hardware-accelerated content into UI elements' (including layered graphics and much improved graphics performance) respectively.
720p HD video recording, HDMI output, textual location-tagging (GPS co-ordinates translated into country-street-city), multi-channel audio, and many other features will all become fully integrated into the platform.
It should be noted that any of these components or features can appear in earlier devices - manufacturers can add custom code and functionality. Indeed this is likely to be a common occurrence. Manufacturers may typically choose to test an addition in their own devices (and use it as a way to differentiate from their competitors) before contributing the necessary enablers to the Symbian Foundation.
Symbian^3 will be functionally complete by week 4, 2010 and will be hardened by week 26. This means is will be available in devices in the second half of next year.
Symbian^4
Symbian^4 is the first release to be defined by the community process (councils, etc.) of the Symbian Foundation. In this sense, it can be seen as the first full release to fall completely under the auspices and planning of the Symbian Foundation and its members.
It is proposed that Qt 4.6 be fully integrated into the Symbian Platform. Further, a new 'Orbit' extension library for Qt, consisting of "more than 50 widgets tailored for mobile user experience" will replace the AVKON UI layer of previous releases; along with this is 'Direct UI' interaction and logic. Together, these also mean a rewrite of the application suite to take advantage of "Qt APIs, Orbit widgets, and Direct UI". Initially this would be for touch and hybrid devices, but this will likely change in future Symbian releases.
This means we will see Symbian^4 ship with a new UI for touch-enabled and hybrid devices based on the Qt framework. The engineering effort for this will be carried out by Nokia. Clearly there will be something of a compatibility break for applications, but, at this early stage, it is too soon to assess their impact (e.g. we could see an AVKON to Orbit compatibility bridge).
Nor does it preclude another company or Symbian Foundation contributor creating an alternative application framework and UI layer (e.g for low end mobile phones or non-mobile phone devices).
Symbian ^4 will be functionally complete around the middle of 2010 and will be hardened by early 2011. This means it will be available in devices in 2011.
It is worth pointing out that Symbian^4 is still being roadmapped and specified, which means the proposals above have not been formally accepted.
Glossary
AVKON - is the technical name for the UI-specific application framework layers implemented on top of Symbian OS. Effectively it's the S60 UI.
ORBIT - is the proposed Qt extension library consisting of around 50 or so 'mobile UI widgets' that make up a UI and will supercede AVKON (S60) as the UI layer on Symbian OS.
Qt - is a cross-platform application development framework. See Qt Software.
EPL - Eclipse Public license, the source code license chosen by the Symbian Foundation for its open source.
SFL - Symbian Foundation License, an interim license being used by the Symbian Foundation for code which has not yet been open sourced.
We'll be covering all the Symbian releases in more detail in the near future.
Rafe Blandford, All About Symbian, 1 May 2009