PDAs are much more than address books; some handhelds are phones, internet devices, and video players.

PDAs can store thousands of phone numbers, appointments, tasks, and notes. All models can exchange, or synchronize, information with a full-sized computer. To do this, you connect the PDA to your computer with a cradle or cable. For models that run on rechargeable batteries, the cradle doubles as a charger. Infrared, Bluetooth, and Wi-Fi (wireless) let you synchronize with a computer without wires or a cradle.

Most PDAs can be made to work with both Windows and Macintosh computers, but PDAs with the Pocket PC operating system usually require third-party software for Macs. PDAs with Wi-Fi (wireless) capability can access the Internet. Many of those without can with the addition of a separately purchased modem. Most PDAs can record your voice, play videos, display digital photos, and hold maps, city guides, and books.

Most PDAs on the market are the familiar tablet-with-stylus type that feature a squarish display screen, a design pioneered by Palm Inc. Today the main choices are models that use the Palm operating system (OS)—mostly Palm models—and Pocket PC devices from companies like Dell. The latter use a stripped-down version of Microsoft Windows. A few PDAs use a proprietary operating system. Blackberry, Hewlett-Packard, Motorola, Nokia, Palm, Samsung, and Sony Ericsson offer units that combine a cell phone and a PDA, often referred to as smartphones.

Palm OS systems. Equipped with software to link with Windows and (for Palm-brand units) Macintosh computers, Palm units and their clones have a simple user interface. You use a stylus to enter data by tapping an onscreen keyboard or writing in shorthand known as Graffiti. Some models have a tiny tactile keyboard. Or you can download data from your computer.

Most Palm OS-based PDAs can synchronize with a variety of desktop e-mail programs, such as Outlook Express and Eudora. (Palm models with VersaMail software are good at handling e-mail with attachments.) And all include a basic personal information management (PIM) application. Palm OS units are easy to use, although navigation between different programs is cumbersome because of the operating system's “single tasking” nature.

Most models make it difficult or impossible to replace the battery yourself. And beyond the warranty period, you can't be sure the manufacturer will do it for you.

Most Palm OS-based models have expansion slots that let you add memory or attach separately purchased accessories. All Palm OS-based PDAs can be enhanced by adding third-party software applications. The more free memory that a model comes with, the more software it can accommodate. There is a large body of Palm OS-compatible freeware, shareware, and commercial software available for download at such sites as www.palmgear.com. Many Palm models come with Documents To Go—word-processing and spreadsheet software similar to that used in Pocket PCs but more versatile. Price: $100 to $400.

Palm's top-of-the-line model, the LifeDrive, combines a 4-GB hard drive with many of the best features of the Pocket PC and Palm OS operating systems. When it's connected to a Windows PC, you can drag and drop files to the LifeDrive's hard drive, even on PCs that don't have Palm's desktop software installed. It's also handy for storing photos from a digital camera that uses a MMC/SD (MultiMedia/SecureDigital) memory card and for listening to MP3 music.

Pocket PC systems. These resemble Palm OS-based models but are more like miniature computers. They have a processor with extra horsepower and come with familiar applications such as a word processor and a spreadsheet. Included is a scaled-down version of Internet Explorer, plus voice recording and perhaps some financial functions. The included e-mail program handles Word and Excel attachments easily.  Also standard is an application that plays MP3 music files, as well as Microsoft Reader, an eBook application.

As you might expect, all the application software included in a Pocket PC integrates well with the Windows computer environment. You need to purchase third-party software to use a Mac. And you'll need Microsoft Office programs such as Word, Excel, and Outlook on your computer to exchange data with a PDA. Pocket PCs have a color display and rechargeable lithium-ion batteries. Unlike most Palm OS-based PDAs, replacing the battery of most Pocket PCs is usually straightforward. Price: $200 to $600.

Ultra Mobile PCs (UMPCs). These are small, fully functional computers that run a version of Microsoft's Windows XP. An early model, the Samsung Q1, released in 2006, has a 7-inch display, measuring 51⁄2 inches x 9 inches x 1 inch and weighing 1.7 pounds. It lacks a built-in keyboard and costs about $1,100.

Whichever operating system your PDA uses, you might need to install programs in your computer to enable the PDA to synchronize with it. This software lets you swap data with leading PIM programs such as Lotus Organizer or Microsoft Outlook.

All PDAs have the tools for basic tasks: a calendar to keep track of your appointments, contact/
address software for addresses and phone numbers, tasks/to-do lists for reminders and keeping track of errands, and a calculator. A notes/memo function lets you make quick notes to yourself. Other capabilities include word-processing, spreadsheet, and e-mail functions. A voice recorder, which uses a built-in microphone and speaker, works like a tape recorder. MP3 playback lets you listen to digital-music files stored in that format, and a picture viewer lets you look at digital photos. A few models also include a built-in digital camera and keyboard.

A PDA's processor is the system's brain. In general, the higher the processing speed of this chip, the faster the PDA will execute tasks—and the more expensive it will be. But higher-speed processors may require more battery power and thus deplete batteries more quickly. Processing speeds are 200 to 624 megahertz (MHz), and models typically have 16 to 256 megabytes (MB) of user memory. Even the smallest amount in that range should be more than enough for most people.

Nearly every PDA offers an expansion slot for some form of removable memory card: CompactFlash, MultiMedia card (slots also accept SecureDigital cards), or Memory Stick. Models with two expansion slots can accommodate a peripheral device, such as a Wi-Fi wireless networking card and removable memory. If you plan to transfer photos from a digital camera to your PDA, make sure the two devices use the same type of card.

Some PDAs offer wireless connectivity. Models with a capability known as Bluetooth can connect wirelessly over short distances to a properly equipped computer or peripheral such as a printer or modem. Models with Wi-Fi can connect over medium distances to a Wi-Fi-enabled home network or to the Internet at “hot spots” in certain airports, coffee shops, and hotels. A PDA combined with a cell phone can make voice calls or directly connect to the Internet via a wireless Internet service provider. It's possible for a single PDA to have more than one of these types of wireless connectivity.

Consider your ties to a computer. Pocket PCs provide a Windows-like interface that allows simple PC-to-PDA file transfer with drag-and-drop capability. They're also more convenient than Palm OS models for setting up a Wi-Fi (wireless) e-mail connection. Most have replaceable batteries along with accessible flash memory to which you can back up data.

Palm OS models run a wider range of third-party software applications than do Pocket PCs. For the basics, they're still easier to use. While most PDAs can sync with Macs, only Palm models do so out of the box. Programs such as Missing Sync and PocketMac support Palm OS PDAs from Garmin and others, as well as Pocket PCs that run older versions of that operating system, such as the Asus A730W. Currently, neither Missing Sync nor PocketMac support PDAs that run Microsoft Windows Mobile 5.0. Both are priced under $50.

Small size vs. extra features. As a rule, a model with a larger display or physical keyboard won't be the lightest or smallest. A PDA with two slots for memory and peripherals is more expandable but will tend to be larger.