Symbian OS is an operating system installed in some smartphones. With such a phone can write e-mails, store pictures etc. However probably the most important thing is that Symbian OS is an open platfom meaning that you can download an SDK and write your own software for such a phones.
The native language for Symbian OS is C++ (the Symbian OS phones supports Java J2ME MIDP and there are some tools to support other languages but C++ gives you access to more phone functionality. Most of the software that comes with the phone is written in C++).
Symbian OS contains features you could except from an operating system (e.g. a file system, drivers for Bluetooth, IrDA etc.) but it does not contain a User Interface (UI). Only some basic UI libraries are standard among all Symbian OS phones. Other libraries depends on the type of the device. There are several Symbian OS platforms which provides a UI library, some other usefull libraries and some standard applications. An application that uses features (e.g. the UI) from one platform will need to be ported to run on another one. However it is said that in a typical application 80% of the code is platform-independent. The platforms are:
Series 60 - this platform was developed by Nokia but it is also licensed to other manufacturers. These phones features a 176x208 display and are referred as voice-centric (i.e. mainly used as a phone) with informations capabilities. They can be operated with a single hand - there is no pen input. An example of a Series 60 phone is Nokia 6600. The UI library for Series 60 is called Avkon.
A new version of the platform (Series 60 2nd Edition with Feature Pack 3) includes the support for bigger screens (208x208, 240x320 and 352x416)
UIQ - these devices features a bigger screen (208x320 with the flip open) and a pen input. They are refered as informations-centric with voice capabilities. An example of a UIQ phone is the SonyEricsson P910. UIQ is the name of both the platform and the UI library.
A new version of the platform (UIQ 3.0) supports smaller screens including support for single-hand operated phones.
Series 80 - this software is used in the Nokia Communicators. Series 80 v1.0 was used in Communicators 92xx. A new version 2.0 is used in the new phones Nokia 9500 and Nokia 9300. They have a big 640x200 display.