UIQ (formerly known as User Interface Quartz) by UIQ Technology is a software platform based upon Symbian OS. Essentially this is a graphical user interface layer that provides additional components to the core OS, to enable the development of feature-rich mobile phones that are open to expanded capabilities through third-party applications.
Read more about UIQ: UIQ Programming Languages, UIQ Iterations Since Symbian V9.1, List of Some UIQ 2.x Phones and Its Version, List of UIQ 3.x Phones and Its Version, First Phones To Use Platform Versions, UIQ Acquisition, End of UIQ