Symbian is a venture originally formed by Nokia, Ericsson, Motorola, and Psion to create easy to use operating systems for wireless devices and personal digital assistants (PDAs). Their first operating system was called EPOC.
Symbian is now primarily used as the underlying OS for two major smartphone platforms: S60 and UIQ.
Software for S60 is generally not directly compatible with UIQ nor vice-versa. However, since both platforms use Symbian, that makes it easier for developers to port S60 applications to UIQ and vice-versa.
Symbian OS underwent major changes in version 9 that make all previous software incompatible. Platforms based on Symbian 9 and later - including UIQ 3.0 and later, and S30 3rd Edition and later - require updated software versions designed to work with Symbian 9 or later.