A smartphone is a mobile phone with such advanced features that make it even comparable with personal computers. Most smartphones have built-in camera, colour graphical user interface, capable of sending/receiving SMSes, MMSes, e-mails, act as personal organizer, MP3 player. These devices are often referred to as converged devices as there are more and more features built into a smartphone that makes it unnecessary to carry separate gadgets (e.g. MP3 player, radio, e-book reader) while moving.
Unlike a PDA, where input is typically pen-based using a touch-screen, smartphones usually have a standard phone keypad for input. And unlike PDAs, smartphones are voice-centric with some added PDA functionality and not the other way around. As opposed to feature phones, smartphones are much more powerful in terms of openness, supported functionality.
Smartphones are open phones, which means that 3rd party software can be installed on these devices. There are a few popular mobile operating systems that run on these devices, like Symbian OS, Windows Mobile, Palm OS, mobile Linux just to mention but a few.