Automotive: The most typical use. Great for the driver that goes to unfamiliar locations all the time. Shortest distances or fastest routes can be calculated. Traffic updates are a useful optional item and can greatly decrease the time it takes to get from point A to point B
Motorcycles: Similar to automotive units but they are optimized for left handed use with large buttons and other glove friendly features
Mobile Phones: A popular add on to mobile phones they have the functionality of a stand alone unit and attach to data ports on the smart phone.
Laptops and PDAs: You can attach a GPS receiver to your laptop and have the data go straight into your hard drive. You can manipulate this data and add data from multiple receivers if you please. No need to have a stand alone unit if you have a receiver attached to your laptop.
Running: Not only do they record your jogging paths some have heart rate monitors and the ability for you to compete with your previous work outs.
Bicycling: Along with the hardware to attach it to your handle bars a cadence sensor and your power output in watts can be used as you ride to gauge how hard your work out has been.
Handheld: The basic unit of the GPS. Basic units can be extremely simple with no maps and just a heading indicator and waypoints to locations you set. But if its features you want they can have colored backlit maps and be water proof along with lots of other features.
Dog Collar: Mainly for hunting, it can track your dog and display it's location and path history. The hand held unit can track up to 10 dogs.
Fishing / Boating Great for plotting a course to your favorite fishing spot. Some come with chart plotters and sounders to locate fish. Advanced units can receive FM broadcasts and can show you the weather.
Flying: Either built in or hand held these units update at a much faster rate to give you a better "real-time" location indicator.
Even if you've picked out the type of GPS for you be sure to look at the features it includes. For instance there are some automotive GPS units that don't say street names. Some units have that capability some don't, be sure to check if you prefer one type or another.
Voice: Very useful when your on difficult roads and can't take your eyes off where your going. The GPS will talk you through the twists and turns of the road to lead you to your destination.
Touch Screen: Typically more intuitive than buttons. Easier to use and can lower the size of the unit since there doesn't need to be any surface area devoted to buttons. All though there are many benefits to a touch screen if you wear gloves while driving (like for a motorcycle) it may be harder to navigate through the menus of the unit perhaps bigger buttons would suit better.
Display Size: Typically the self contained units will be around 4 inches by 3 inches. Easy to hold in your hand or attach to the inside of the car. There are smaller more portable ones for hiking / backpacking. There are also larger ones meant to stay attached to a vehicle. You can get almost any size screen with your GPS if you decide to attach it to a laptop or PDA
Maps: Does this GPS come with all the maps of where your going to use this device? are they able to add new maps if you so choose?
Preset Waypoints: Some come with preprogrammed waypoints like stores and shops. Do you travel a lot in unfamiliar areas? Perhaps this feature would help you find the next hotel or gas station and some even have the phone number of the establishment with the location.
Other Applications: MP3 player, Picture Viewer, currency converter, Bluetooth. Make sure the model you select has the applications right for you.
Accessories: Added Antennas, Heart rate monitor, even extra Dog collars.