PDA stands for Personal Data Assistant.  This is a generic term for any handheld or palm-sized device that uses a processor chip and is capable of launching  different programs that perform divergent tasks, such as text editing, basic spreadsheet functions, digital graphics display, and digital sound recording and playback.  Increasingly, PDAs are expected to send and receive email and provide at least basic internet browsing capabilies when supplied with wireless network capabilites.

Things to look out for when buying: It is very rare for a seller to try to burn you, but sometimes sellers simply don't know what they've got and how to evaluate it.  You need to know that the device syncs with a desktop system.  If the description says it comes with a USB cable or serial cable but does not specifically say that the sellers knows it will sync, ask.  If the device requires a serial cable to sync it may only sync with certain versions of Windows (95, 97, 2000) and may require early versions of MS ActiveSync.  Some newer laptops do not have serial port connections. You can get around these potential problems if the device has an infrared port, so ask about that as well, if it is not mentioned.  Note that you need an IR port on your desktop or laptop system as well to take advantage of the feature. 

If the description mentions a storage card slot but does not specify the type, ask.  Earlier PDAs take now-obsolete cards that are to the SD card what the floppy disk was to the CD.

Usually the first thing to go on a PDA is the battery or the backup battery.  There's really know way to accurately assess battery status until you have the item in hand.  The seller may quite correctly state that the battery holds a charge, but if it only holds a charge for two hours, it is on its last leg.  Primary batteries are easy to replace but check availability for older units. Replacable backup batteries ("Coin" types) are best since they are cheap and easily replaced.  Some models use a permanent backup battery that is really just a ceramic condenser.  It can be replaced but it is a minor hassle that requires some technical skills special tools or shipment to a specialty shop.  If in doubt ask very specific questions about the backup battery type and any messages that may appear regarding battery status when the unit is powered up.

Genreal stuff:

The variety of functions available on PDAs has increased exponentially in the last five or so years.  What you need in a PDA depends on how you want to use it.  Some fundamental differences exist between PDA operating systems (O/S), the underlying computer language that enables the programs a device runs.  Most PDAs today use a version of the Palm O/S or the Windows Mobile O/S.  The latter is a Microsoft product, the former is not.  Consequently, if you want to run the mobile versions of Microsoft Office products, such as Word, Excel, and Powerpoint (despite endless promises, there is no mobile version of Access or any other truly relational database), and have seamless drag and drop capability between your desktop and mobile system for those applications, you should look for a device that uses the Windows Mobile O/S.   However, PDAs using the Palm O/S tend to be less expensive, have a suite of mobile applications similar to most Microsoft products, and usually provide translation software to make downloaded files compatible with MS Office counterparts.  Microsoft and third party vendors provide a number of robust programming languages for the Windows Mobile O/S. Programming capabilities for the Palm O/S are less robust.

PDAs have become increasingly loaded down with "bells and whistles" such as voice recorders, digital cameras, MP3 players, etc. Every added function adds a few dollars to the price. If you really want to take/view digital pictures or video on your PDA then the cost of a higher end item is reasonable.  And of course if you need to tap into Federal satelite networks and download schematic diagrams of secret weapons to CTUso they can be linked into Special Agent Jack Bower's cell phone, you need a very robust PDA.  On the other hand, if you want to edit text or review and edit spreadsheets while riding the local bus to the Park-and-Ride and sync your edits to you laptop files when you get home, the lowest-priced PDA running Windows Mobile 2003 or later or Plam 5.0 or later will probably exceed your needs.

Some things to consider in general:  the main constraints on PDA performace are screen size and resolution and memory capacity.  Screen size, brightness, and resolution are deirectly related to battery life as the screen is the most amp-hungry part of the PDA.  Consider the trade-off between battery life and screen size depending on your needs.  Most PDAs come with between 32 and 64 megabytes of ROM.  The amount of ROM affects both the speed of applications, particularly those that display graphics or video, and the amount of information that can be stored on the device.  If you are working primarily with small text files and spreadsheets, 32 Mb is a lot of memory and a lot of performance speed.  If you are using a lot of bitmap or other graphic images, video, or sound clips 64 Mb may not be adequate.