The basic GPS device determines your location based on signals that it receives from satellites. But just knowing your latitude and longitude coordinates isn't all that helpful without a map, so GPS devices also include large databases of map information built in. Because they know where you are, where you're going, and what roads lie in between, they can map out a very detailed route for you.

GPS devices are quite versatile in how they deliver your information. They all provide a map onscreen that looks like a conventional paper map, usually with your route and position highlighted. You can zoom in and out for detailed views and the big picture. Or you can switch to a view that looks as if you're driving in a game or one from overhead.

GPS devices also can convey directions using turn-by-turn voice instructions. If you've ever been in a car with a GPS you've heard the lovely, well-modulated voice of the GPS unit, telling you where to turn next.

In the last few years, GPS has extended its reach with all sorts of new features. Color displays, selectable voices, waypoints on the road to where you're headed—and that's only the tip of the iceberg. Some GPS devices can now read the names of streets aloud to you. Others have 3D maps that make it seem as though you're navigating in a video game. Others have built-in walkie-talkie radios (a favorite with hunters), music and video players, or the ability to answer cell phone calls. Many are starting to carry information about local points of interest or shopping. Best of all, you can get a solid GPS for as little as $80, although the more exciting and versatile units will cost a few hundred.

So what makes one GPS different from the next? A few things:

Size and Detail of Map Database: The maps that fuel each GPS device come from a few reliable sources. Most systems will come with a small number of maps and then offer you the opportunity to buy more.

Display Screen: More expensive units will have a color screen that's at least four inches, in a slim casing, and with good resolution. Many even let you store and view your photos.

Out of Car Experience: The majority of GPS devices work in the car. A good number will work on city streets, though they might give you directions as though you were a car (obeying one way streets). Some specialized GPS like the Garmin Colorado have additional sets of maps (like topographical maps) and are good for walking, hiking, or boating.

Local Search: Many of the latest GPS store local information, everything from locations of ATMs to restaurants. Movie listings, fuel prices, news and stocks, local events, weather reports, and other useful information are all provided. Google, Yahoo!, and MSN Direct are three services that feed data to various GPS devices.

Send to GPS: This feature lets you plan trips and look up destinations from the convenience of your computer and then wirelessly send to your device.

Voice Input: A few new GPS are completely operable via voice, searching for addresses and providing route information without requiring the driver to remove their hands from the wheel. Typically, a remote device attaches to the steering wheel and activates the voice commands.

Real Time Data Reports: Other users of the same GPS as you will be able to share their traffic information with you.

Here are a few exemplary GPS models. Remember that once you buy the device you'll still incur extra fees for some monthly services.

Garmin nüvi 880 is a top of the line (and top of the line-priced) portable navigation device with speech recognition, dynamic MSN Direct content for events, news, and local attractions, and an impressive array of other convenience and safety features. It's got a bright 4-inch display with 480x173 resolution. Garmin makes a range of other great GPS from $400 and up.

Magellan just introduced the Crossover GPS, which can be used for in-car navigation and for hiking and other on-foot outdoor travels. It contains topographical maps for all 50 states, Canada, Puerto Rico, and Mexico and retails for $549. The newest Magellan Elite 5340+GPRS ($1,200) offers information through Google Information Services, much like the Garmin nüvi 880 offers with its MSN Direct. Plus it lets users share live traffic data from their devices. Several other GPS device makers, including Dash Navigation's Dash Express, have similar features.

TomTom GO 920 has what it's calling "Enhanced Positioning Technology." It uses gravity and movement sensors to help calculate your position if satellite signals become unreliable. Great for driving in tunnels. It's also got a "help me" safety button to find a hospital or contact the police, as well as a MapShare feature that lets you share your map corrections with the TomTom community (from $450).

Maybe they're tired of fighting about which way to turn, or maybe they're feeling a bit fragile and lost, but this holiday, GPS is topping many shoppers' lists. Amazon Electronics, the bellwether of who's buying what, just sent me a list of its top 25 most-purchased items last week. The Garmin Nuvi, in various models, appeared four times on the list. (Other repeated top sellers included Canon and Apple.)

Why the run on GPS now? The technology is finally mature enough that it really can get you where you need to be and keep you safe while driving at the same time. The price is reasonable enough. In addition, it's become a one-stop dashboard topper that can tell you where you are or where you want to be (as in the nearest gas station), let you talk wirelessly to others, or entertain you with photos, music, and more. And the new bells and whistles just keep getting better

One of the reasons Garmin is so popular is that there's a perfect unit for everyone. (Or, looking at it another way, you may find the 35 different models of Garmins available just for automobiles a bit mind-boggling.) According to numerous shopping reports, Garmin's entry-level Street Pilot 550c was a huge seller on Black Friday.