PDA is short for Personal Digital Assistant, Just as its name implies: it is a handheld device that combines computing, telephone/fax, Internet and networking features all in one. It not only can be used to manage personal information (such as address list, plan, etc),what's more, you can surf the internet with it, receiving and sending Email or fax, even can be used as a mobile phone, most importantly, these functions can be realized through wireless mode. Of course, it doesn't mean that all the PDAs have all these functions, even if they have, maybe it can not achieve it due to the lack of corresponding service.
Compared with the traditional computer, PDA is light and compact, high movability, and yet with powerful functions A typical PDA can function as a cellular phone, fax sender, Web browser and personal organizer like working directory, address book, card exchange and scheduling etc.. Unlike portable computers, most PDAs began as pen-based, using a stylus rather than a keyboard for input. This means that they also incorporated handwriting recognition features. Some PDAs can also react to voice input by using voice recognition technologies. PDAs of today are available in either a stylus or keyboard version.
In recent years, due to its increasingly powerful function, PDA chip has some new development, it generally integrates electronic dictionary function, the expansion of screen size, handwriting recognition technology, and it could be uploaded to and synchronized with PC data

Classification :
PDA actually should be subdivided into 4 categories those are electronic dictionary, palm computer, handheld computers and personal communication assistant.

Features:

1.  Simple in structure, usually there is no operating systems, or operating system and application software are combined together.

2. Usually has a cute and fashionable appearance, and generally it is small.

3. Easy to use, not in pursuit of complex function, just simple query, records and the remind function.

4. It can not expand and update software owing to the cost limitation

Operating systems

There are three major players in this arena:

• Windows CE
• EPOC/Symbian and
• Palm OS.