Portable GPS units are small enough to be carried between the house and the car or between different cars but not small enough to fit in a pant or shirt pocket. They typically weigh about 10 Oz and are about 4" in width and several inches in both height and depth. They range in price from $250 for a basic unit to over $500 for one full of the latest bells and whistles. The biggest brand by far is Garmin which makes the StreetPilot Series of portable GPS units.
Pocket GPS units weigh around 5 Oz and are small enough to fit in the palm of your hand and also a pant or shirt pocket. They typically have a 3.5" screen size and cost more than the Portable types because of the small form factor. They start at around $400 and can cost over $600 for a full featured unit. The biggest brand by far is Garmin with the Nuvi series of GPS units.
Automobile In Dash
An automobile In Dash unit is built into the dash of the car and so it is not portable. More and more new cars offer In Dash GPS as an option but they are usually expensive for what you get in terms of functionality. The advantage is that they are integrated into the dash instead of mounted on top of the dash and they aren't easily lost like a portable unit. Pioneer makes a very good In Dash GPS unit.
Fish Finder GPS units typically have a dual function of chart plotting and sonar technology. The chart plotting is used for identifying shorelines, roads, and surface features and the sonar is used for locating fish. The most popular brand of fish finder GPS units is Humminbird.
Camping and Hiking
These GPS units are designed to be lightweight and rugged for outdoor use for hiking, camping, and geocaching. Some of the more expensive units are also waterproof. They usually come preloaded with some maps but you will often have to load additional maps from your computer to the device if you are heading 'off road'.
Fitness and Cycling
Fitness and cycling GPS devices are used for walking, jogging, running and cycling. Typically they fit on your wrist like a watch and can track speed, distance, pace and calories burned. Cycling GPS units often fit on the stem or handlebars of a bike.
GPS units designed for motorcycles are very similar to those used for cars except that they are often waterproof, vibration resistant, and designed to fit on a motorcycle console. Bluetooth hands free technology is also popular for these devices.
Marine GPS units typically have chart plotting functionality. They are loaded with a special marine database including marine navigational aids like buoys, sound signals, day beacons and tide data.