GPS means Global Positioning System.
A number of satellites orbit the planet and transmit signals which are picked up by GPS devices.
These GPS devices translate the satellite signals into your accurate location coordinates for various activities such as hiking and walking.
A hiking GPS is small handheld unit used to aid navigation.
There are many makes and models of hiking handheld GPS devices on the market to choose from.
Considering the following physical features will help you to arrive at a choice that is best for you:

• Display Screen - The display screen is part of the front of the case and will normally be made of a sturdy plastic material.
  Screen display areas can vary significantly, from the smaller basic coordinate black and white screen of an entry-level device to the larger ones that display 3-dimensional full colour maps.
  Your choice of screen will therefore usually be determined by the type of advanced functional features you want from your GPS unit, which in turn is determined by the type of navigation support you want.
  
• Power Source - The power source for a hiking GPS is usually batteries which are either replaceable or not.
  If the batteries are not replaceable then they will be rechargeable but fixed and the case will not usually be required to be opened.
  If the batteries are replaceable then the rear of the GPS case usually opens up to reveal the section for the batteries, which may be either throw away or rechargeable.
  The battery section should be waterproof to prevent damage by water and other debris to the inner parts.
  If the batteries are not replaceable then you are restricted to the length of charge within one battery unless you take a portable power supply along with you to recharge the device.
  On the other hand, a device within replaceable batteries has the advantage of a longer usage time as extra batteries can be taken along to replace the used batteries.
  
• Weight - If you want to reduce the overall weight of your hiking equipment then the weight of your GPS device may be a factor to consider.
  However, you will generally find that the lighter the weight of the GPS device the smaller the device is likely to be with fewer and smaller features.
  As an example, a smaller, lightweight GPS device may have a small screen with minimal advanced functional features.
  In my opinion, the weight of a hiking GPS should not really be that much of an issue for most hikers as most of them are relatively small and lightweight anyway.
  However, ultra lightweight hikers may disagree with me.
  
• Rubber Gasket - A rubber gasket is usually used to form a waterproof seal between the front and back of the GPS unit.
  Not only does it also protect any computer cable connector, it serves as a good grip for the user to facilitate using the device with just one hand; and as protection against some knocks.
  The majority of GPS devices dedicated to hiking are also waterproof and robust enough to withstand some rough treatment.
  

Entry level hiking GPS devices are usually cheaper with smaller black and white display screens, lighter and have a longer life power source.
However, they only provide you with your position in the form of a map grid reference and you will need to use a paper map and compass to navigate.
Conversely, the more expensive higher end hiking GPS devices have larger display screens which are often colour so that they can be more easily used to navigate with.
The colour screens of the higher end devices are provided to support their greater functionality such as greater memory storage for route waypoints, digital mapping and instruments such as a compass and altimeter.
Some even incorporate two-way radios.
As a consequence of these extra functional features the power source required by the devices need to be larger and heavier and/or they may have a shorter life.