A personal digital assistant (PDA) is a handheld computer, also known as a palmtop computer,have both color screens and audio capabilities, enabling them to be used as mobile phones, (smartphones), web browsers, or portable media players. Many PDAs can access the Internet, intranets or extranets via Wi-Fi, or Wireless Wide-Area Networks (WWANs). Many PDAs employ touch screen technology.

The main purpose of  PDA  is to act as an electronic organizer or day planner that is portable, easy to use and­ capable of sharing information with your PC. It’s supposed to be an extension of the PC, not a replacement.

Software typically required to be a PDA includes an appointment calendar, a to-do list, an address book for contacts and some sort of note program. Connected PDAs also typically include E-mail and Web support.

Types of PDAs :

Palm PDAs
Most Palm devices are made by palmOne, which offers the Zire and Tungsten product lines. The company formed in 2003 when Palm Computing acquired Handspring, Inc. Sony, which produced the Palm-based CLIE, stopped producing PDAs in 2005.

Pocket PC the generic name for Windows Mobile PDAs

All PDAs come with some kind of personal information management (PIM) software that typically handles the following tasks to keep you organized:

• Store contact information (names, addresses, phone numbers, e-mail addresses)
• Make to-do lists
• Take notes
• Track appointments (date book, calendar)
• Remind you of appointments (clock, alarm functions)
• Perform calculations

PDAs can run specialized software applications:

• Windows Mobile devices come with Pocket versions of Word, Excel, Internet Explorer and Outlook (includes e-mail and PIM functions), along with Windows Media Player and voice memo recording.
• Most Palm OS devices include applications such as DataViz Documents to Go (compatible with Microsoft Word, Excel, and PowerPoint), palmOne Media (for photos and video), VersaMail e-mail software and web-browsing software.
• All types of devices can run other kinds of software including games, multimedia, expense, diet and exercise, travel, medical, time and billing, and reference.