A smartphone is any electronic handheld device that integrates the functionality of a mobile phone, personal digital assistant or other information appliance. This is frequently achieved by adding telephone functions to an existing PDA or putting "smart" capabilities, such as PDA functions, into a mobile phone. A key characteristic of a smartphone is that additional applications can be installed on the device. The applications can be developed by the manufacturer of the handheld device, by the operator or by any other third-party software developer.
It is increasingly difficult to define exactly what qualifies as a smartphone. Almost all new mobile phones have some rudimentary PDA functionality such as phonebooks, calendars, and task lists. Furthermore, BREW and Java ME devices allow for the installation of additional applications but are still not considered smartphones. There are many BREW devices with PDA functionality, the ability to run third-party applications in native code and sporting displays as large as 240x320 pixels; yet they are not considered smartphones. The elusive definition seems loosely tied to the particular operating systems listed below.